Oh, shit, that was exactly what I was looking for.  I figured there'd be such a button, good thing I thought to ask!  :)

... well, I tried it and it's slightly less convenient than what TLS had (in that one it just sent you to the main menu directly, in this one it reboots the entire game so I have to resize the window again), but it's still better than having to reopen the game manually!

AFAIK prices increase increase for Blessed Water (Daily) and Enhance Equipment/Sharpen Sword costs (later on, with armor enhancement, too). For the latter two, cost increases every time you do the respective action, rather than as a daily progression.

For the Hydrangeas, you may have to use a bomb or two (have John use them so both fighters can get criticals). If you can't kill at least one of them before they use the pollen fog move (which is the precursor to mind pollination), you might as well put the fight off until you can handle it later.

Just as an FYI, you technically can wait the debuffs out - the Mutated Hydrangea's 300 MP will tap out after casting skills 30 times (Foggy Pollination, Mind Pollination, and Draw Power: Earth all consume 10 MP), but be prepared for a long slog if that's the way you're going with it.

Also, if you haven't already you probably should pick up Pierce from Paul as it will boost your damage output.

Ed: The "split" demons only have 1 hp - the ideal situation is to have John use Slash the same turn the splits, but if John using Slash is completely nonviable you could theoretically make due with a Stasis Bomb as it will hit all enemies (though you'll probably want to keep a few for a later boss fight).

Tailwind dispels the Hydrangea accuracy debuff on melee attacks,* and spells(fire/light) are better than bombs if you have the MATK for it since their attack debuff doesn't reduce MATK.

Ogre Commander is tanky, but Ogres are extra vulnerable to being attacked from behind, his attack pattern is/was very predictable(attack, attack, windup, defense debuff IIRC), and his damage isn't very threatening if he can't land his windup martial to debuff your DEF. The Goblin add is also weak to just about every type of magic, so killing it with fire is a cheap way to get tactical advantage.

The Low-Demon's clones have 1 hp each(2 on nightmare). A single AoE attack(e.g. slash) by John or Aura should bring the 5 back to 1.

*I think storm is supposed to do this too.

To play devil's advocate for a bit: there's a bunch of spells you probably want to learn anyway, ATK is also a pretty massive money sink, the Slime can be built as a pretty strong melee attacker and Defensive Stance does not reduce MATK so mage Aura tends to be a pretty tanky Aura against physical fighters.

I think the game is trying to be balanced around the idea of your Slime filling the role Aura isn't filling. Dueling becomes an absolute pain on mage runs though.

At to the optimal use question: 

Probably best to either sell the living hearts or feed them to the Slime(materials fed to the slime give EXP based on their gold value) with selling winning out as far as I'm concerned. DEF/MDEF/Resistances  tend to matter more for survivability than HP since good fights are the ones where you can stat-check hits into single digit damage and honest hard work at the blacksmith(or drugs if you need them) is a better way to bump Aura's HP. Pyromantium and Ether are best saved for when you unlock Verdeaux since their stat gains aren't great and the workshop gets more expensive every time you enhance equipment.

When you get the Slime early on it is kinda cruddy (but still useful for the Darry+Barry fight), but as long as you've been feeding it, once you hit mid/late game it's stronger than Aura (excepting the fact that it has a Darkness weakness). It's Wind form is also practically a prerequisite for underwater combat. The Slime is also viable for the Alchemist when you get the Morph (Fire) skill at Slime Lvl 5. Note that the slime has to be unsummoned when you talk to the girl who taught you the summon spell, the game is not great about telling you this - I honestly got the Slime to lvl 7 before I figured that out on my first run because I practically always had the thing summoned.

One caveat with it being stronger though is that the Slime is definitely a lot better with the Bonding 1-3 passive (involves lewd scenes, will drop Victory Score).

Hm? I'm pretty sure as long as you route around it there shouldn't be a quest that's directly tied to the White Oak. If you are talking about the Darkness In The Woods quest, that's for a Low Demon that's further back in the forest. White Oak's pretty dang strong (and not necessary for progression), I wouldn't really recommend fighting him unless you can 1 or 2 shot the saplings with a critical Slash.

Call me crazy but I don't think there is a quest for the Whiteoak? Which quest is this?

pathetically easy

I chuckled, sensibly. I assume that guide (the one on the wiki?) has a perfectly minmaxed savescummed Aura or just got real good RNG. The quest giver pretty much tells you Whiteoak is tougher than the demon.

yeah, its possible. you should reload a save prior to that fight and beat it without poison, as pierce 2 is only obtainable from doing it

It is possible to get to the Underground City on your own - you don't absolutely need the demon worshippers to trust you. Once you find the entrance it's just a matter of entering a second time with your waitress uniform to bypass unnecessary mob encounters. I mean, sure, the enemies do drop gold, but I'm pretty sure you're not nearly strong enough to make it worth your while at this point.

Though TBH going the "sneaky route" is probably meant more for a second playthrough.

The fight with Rene allows spells, so while I haven't done it in Nightmare yet, that one struck me as the most straightforward duel in Verdeaux. Speaking in terms of defense, options include using Rock Armor or a Lightning Jam coating, though I would suggest also taking out the Lamia in underground Nephlune if you choose the latter.

Also, maybe it's just because I can't help but compare the art style to a lot of Japanese games, but if you wanted to mentally will Aura to being 23 she looks mature enough for that to me?

Okej, for future reference, here's the strategy on nightmare mode that eventually worked for me:

(first thing is what Aura does, second thing is what Slimey does)

(this is with nasty weapon coating on, unsure if it's necessary)

Turn 1:  Heat Up, attack phantom.

Turn 2:  Heat Up, attack phantom.

Turn 3:  Light I on phantom, attack phantom.  Lamia will switch to Offensive Stance.

Turn 4:  Attack Lamia, throw Flash Bomb.  The attack on Lamia should do roughly 500-550 damage.  (The flash bomb isn't strictly necessary but makes the RNG for the remaining steps less hellish - the Lamia needs to NOT hit Aura, and Slimey won't last long enough if she hits every time)

Turn 5:  Heat Up, Protect (or defend if you're feeling lucky and want to leave it to RNG whether Lamia attacks Slimey or Aura, with the benefit that Slimey would survive at least one hit).

Turn 6:  Light I on Lamia, Protect (or defend, etc)

Turn 7:  Rampage I on Lamia, and what slimey doesn't doesn't matter because either this will finish off the Lamia (dealing 1500ish damage), or it won't (because the Lamia randomly used guard or because you missed due to RNG) in which case you have to reload

In any circumstance, the Lamia choosing to attack Aura is basically instant death.  It spends the first four turns powering up, but after that it's basically RNG whether it chooses to attack Aura or Slimey (or, sometimes, it randomly guards for seemingly no reason).  If you deal enough damage (I think about half health), it will summon its snake friends, which is also basically instant death.  Do NOT use Offensive Stance as the pure run walkthrough suggests, as it doesn't work here (you can do steps 1-4 if the phantoms randomly choose to not attack Aura, but if you attack the Lamia with double-boosted attack with tactical advatange, it deals around 1000 damage, which isn't enough to finish off the lamia but does bring it below the point where it summons its friends).

I advise taking a break - maybe give the demo of Fear and Hunger 2: Termina a try.