Hiya, I've done an editing pass of most of the files. It's broken down by file name, page number and heading.
#file
-page number: heading: 'Single quotes represent a section of book text that needs polish, hopefully enough included to easily and uniquely identify the passage' (brackets represent my comments or fixes 'Single quotes in a bracket are what changes should be made');
-additional section within the previous heading
This is mostly spelling and grammar, but includes style, form, meaning, rules and a bit of layout. I tried to work backwards from your style and keep true to your intent, changing as little as possible of your voice. Excepting a couple places that needed a bit or polish. I'm sure this isn't exhaustive, but at least it's extensive! Hopefully everything is clear and you find it useful!
#customize promo graphic
-'Mix and match over 180 Parts form 31 Classes' ('180 Parts from 31 Classes' (('over 180 Parts form 31 Classes' makes sense, but not with 'Mix and Match' at the beginning of the sentence)))
#metro jungle biome:
-description at the top: 'Sky scrappers' (should be 'Skyscrapers');
-flooding cave: 'emanate blue' (should be 'emanate blue light');
-overgrown district: 'peak out' (should be 'peek out', a building could peak out at a certain height ((such as above or below the trees)), but not from behind something);
-jungle thicket: 'thicket' and 'Thickets' (either capitalize both or neither. Power line thicket has a lower case one too);
-Dense jungle: 'Old-World trees are like a shrub to a mec but a Wyrm-Stalks' ('a shrub' should be 'shrubs', remove 'a' from before wyrm stalks);
-'Wyrm-Stalks' and 'Old-World' (should be 'Wyrm-stalks' and 'Old-world');
-overgrown district: 'Old world office buildings' ('Old-world' for consistency);
-arcade sink hole: 'old world' ('old-world');
-'sink hole' ('sinkhole');
-'have sunken' ('have sunk');
-clearing: 'tall glowing grass' ('tall, glowing grass')
-bashe plant: both counters: 'IF:' ('IF :' missing style space ((later counters tend to be written like this, along with most of the mechanics book, so I assume you defined it as you went on)), on both counters)
-fortune bug: 'on their back, Fortune Bugs' ('back; Fortune Bugs');
-mystery sack: 'Represent Fortune Bugs with a randomly rolled d6' (Is it supposed to be rolled only immediately before they get popped? Or are players supposed to know what number they're popping?);
-blood pitch sack: 'Range 5x5 - Self: Apply
[Stuck]' (since it's the mec action that triggers this, can you choose where to place the aoe and whether to target yourself? Or is it the bug's action?);
-super charged sack: 'Gain an Immediate Free Action' (would be more clear as 'Take an Action Immediately as a Free Action');
-nozuchi: roller spike: 'Range 8 : move in a Straight Line' (This doesn't actually do much, because it has one target in range 8, how far it moves isn't defined and it doesn't call out being able to move through Units. 'Line 8' and repositioning to the farthest empty Space in the aoe would be the simplest solution I think.);
-barbed: 'IF:' ('IF :' missing style space)
-salamander: fire belly: 'Unit is Immune to Blazing' (Blaze, the damage type? or [Burning] the effect?);
-'IF' (should there be a colon here?);
-predatory: 'focus on DMGing (Thus eating) Swarm Units' (this means that any amount of damge dealt to a Swarm triggers Pheromone release? Or only the final point?)q
#midas convoy:
-biome effect: reinforce: 'Phalax Squad' ('Phalanx Squad');
-(is this a full group of 4?)
-midas sand rail heist: 'which has 4 Ballistas' (plural of ballista is technically 'ballistae');
-'This Platform is 12x12 Spaces big' ('12x12 Spaces large' would sound better);
-'If an Allied Unit falls off the Sand Rail without [Flying] while it's still moving, they'll take' (I'd change to 'If an Allied Unit without [Flying] falls off the Sand Rail (and while the Sand Rail is still moving), they'll take' to enhance clarity);
-'Pilots inside MECs survive easily' (which of the midas units are MECs?);
-(the section ends with 'MECs are free to Shift', but the sentence sounds odd and there's no closing punctuation. Did you mean to give a fallen mec a free shift action? Or simply that they can move after a fall?);
-open air railcar: 'no walls and thus Units' ('no walls, and thus, Units');
-wildlife rail car: 'Wyrms in Electro-Cage' ('Electro-cages');
-turret railcar: 'This open-air Rail Car' (all other instances use 'Railcar');
-aquilifier: 'a large variety rapid construction' ('a large variety of rapid construction');
-phalanx squad: coring beam: 'Range Line of Sight' (the core book says this is 'Range Any');
-centurion: flash missile: 'Range Line of Sight' (the core book says this is 'Range Any');
#Hydra Matriarch
-rampage: 'Move 8 Spaces : Units in the path' (This should probably specify that it can move through occupied Spaces);
-regurgitation feeding & two more grow back: (so these specify whelps and hatchlings respectively, healing one unit and spawning the other. Since pg. 37 in core mentions bosses can be capable of engaging a party by themselves, shouldn't the Unit the boss spawns be the Unit the boss heals? ((the hatchlings get ohko'd by burning anyway, so once the players figure that out it shouldn't be a problem)) or maybe it should heal hydra type units in general for future compatibility);
-two more grow back: 'Spawn a Hydra Hactchling Unit beside' ('Hatchling' also 'adjacent' rather than 'beside');
-hydra whelp: better than one: 'gain +1 SD per Hydra-Type Unit beside the ATK's Target' (is this supposed to only trigger for -other- hydra type units? Also, while easily understandable, I don't think 'Hydra type unit' is defined);
-(it should also read 'adjacent' rather than 'beside');
-hydra egg nest: basics: (there's a line break in the middle a separation colon, so the colon begins a line by itself which looks silly);
-(Presumably the hatchlings spawn in the same space the egg nest occupied, and the egg nest is removed. It would be handy to call that out);
-restless hatchlings: 'IF : there is an Enemy Unit adjacent to the Hatchling' (enemy unit refers to the players here? I think it would be good to use another term, maybe 'Pilot Unit'. Allied/Enemy should always be player facing, such as 'allied/enemy phase' which remain as such even if from the enemy's perspective. Perhaps refer by Faction or Biome? 'Non-Hydra Type Unit' or similar would be more generally helpful here, even if it is a bit limiting);
-(Is it supposed to trigger when next to a hatchling swarm, or next to the egg nest? ((My guess is that 'the Hatchling' here is a typo)) Do all eggs on the field hatch when this triggers?);
-hydra young: 'keep the most matured eggs nested in her stomach' ('nested' should be 'nestled')
#fenrir colony:
-bile blast: 'Max Once Per Enemy' ('Max Once Per Enemy Phase' probably);
-swallow the sun: 'IF:' ('IF :' missing style space);
-'if [Burning]... loses 5 VT then removes it' (on pg. 33 of core the [Burning] Status Effect describes enemy units taking 5 direct damage from counters that remove [Burning]. Presumably this is an unintended doubleup. Dread ooze has the same issue with its Combustive counter);
-clever wolf: (says it can split the free shift up on the allied phase, which I think is a misprint);
-dread mold: 'once it grows large enough it'll burst from the walls or oozes out from machinery' ('oozes' should be 'ooze');
-draugr: FEL BEAM: 'FEL BEAM' ('FELL BEAM');
-dread ooze: combustive: '[Burning] is Applied' ('IF : [Burning] is Applied to Dread Ooze');
-fenrir, draugr, and dread ooze descriptions: they're each described as 'molding' or 'molded' (should be 'moldering');
-fenrir desc.: 'sloshing dripping pitch' ('sloshing, dripping pitch');
-'oozing molding symbiotes' ('oozing, moldering symbiotes');
-draugr desc.: 'shambling molded MEC' ('shambling, moldering MEC');
-'oozing pulsing black mass' ('oozing, pulsing black mass')
#august:
-duelist custom: 'Move to Target' (There's no range here to target; if it can affect anyone it should probably say so);
-'For every Spaces of Flight the August goes down' ('For every Space the August descends while [Flying]');
-golden touch: 'Move to Target : Target the MEC with the most SYN' (again, is this line of sight? all the midas mooks specify when their abilities target los and Range Any. Core pg. 32 and mech shop iii both say Range Any is just line of sight anyway. And couldn't the Target spend all their syn on recharges/overshield at instant speed? It might want to call out if it goes on the stack in a special place);
-divine missile barrage: 'IF: no Targets are hit' (this 'IF:' could have a space, but it's the only IF statement buried in an enemy ability rather than at the front, so there's no precedent);
-'to d3 Target on the Field' ('Targets');
-mandate of heaven: 'Range 3x7' (There's no range here either, to place the aoe in; if it can be anywhere that should probably be specified);
-'cleaves the Earths' (either 'cleaves the Earth' or 'cleaves the Earth's crust');
-(also should probably only be once per turn cycle, maybe even every other, unless the only way to beat him is supposed to be make corroding stick/chip direct damage);
-unstoppable ambition: 'IF:' ('IF :' missing style space);
-rubicon protocol: 'IF : The August is down to 40 Integrity' (since this is a counter it should be 'The first time the August is reduced to 40 or less Integrity' or I think it might just trigger repeatedly);
-'or Apply [Burning]' (presumably it should be 'and Apply [Burning].' Also, it should specify if the burning replaces other status effects, or applies them in sequence ((granting the 2 direct damage)));
-magnify: 'use its Swarm ATK at any Range as long as it has Line of Sight' ('use its Swarm ATK at Range Any');
-philosopher's stone: 'Spawn 1 Golden Nanite Swarm' (should probably specify 'in an open Adjacent Space');
-pacifist war prince: 'A bold plan from a price who grew' ('price' should be 'prince')
#mec sheet:
Reminder text inconsistencies:
-mec status: soak: 'negates damage after shields' (core says soak works on shields)
-betyl bond: 'generate d6+2 syn' (core says d6+3 syn)
-uncontainable: '1 Free Shift Action Each Ally Phase' (frame sheet says any phase. also: it should read 'Allied Phase' if this is actually correct)
#pilot sheet:
-stat check: 'you can always roll less' ('you can always Roll fewer');
-roll table: (the reminder box here is pointing to even numbers that generate synergy. but those include risk dice. are risk dice supposed to also generate synergy? I assume not with the pg. 3 core statement that 'dice' refers to skill ((not risk)) dice and that syn is dice -you- roll ((unlike risk dice)). Also, maybe it should mention working on improvised actions);
-status effects: '[Effects]' ('[Effect]s' This one is on the low reminder text verison as well);
-mec actions: strategem: 'Second' ('Secondary');
-shift: 'Includes movng up or down one space if your MEC can [Fly] or [Swim]' (Is this one space in addition to regular speed? Can you only alter your height by 1 per shift action? The core book says any unit can move in any direction and then fall. pgs. 21, 28, 41, 42);
-syn burns: overshield: '[[Overshield]' ('[[Overshield]]');
-overshield and remove effect: 'From Self or adjacent Unit' (core does not say you can affect an adjacent unit with these on pg. 29, but the Synergy tracker sheet reminder text does);
-'[Communicate]]' ('[[Communicate]]');
-strategem guides: 'Second' ('Secondary')
#tension sheet
-dazed & stuck: 'You also Roll with +4 Risk' (core says +2 risk)