Heya Roundkights!
I just have the extra set of keys as a backup just in case you accidentally rebound your keys to something uncomfortable while navigating the menus. Then you'd be able to rebind your keys using the backup. (The ones displayed on screen)
In terms of the 'no backtracking', that was intentional to try and keep the pace of the game moving forward. In the completed game it will have more unlockable movement mechanics to help with getting the collectibles. For example, one of the upgrades allows you to grab ledges in the current non-demo version of the game.
I'm unclear on what you mean with the intangible floors/walls. Some of them are intended to hide secret paths to reward you for exploring and the forest area the darker green tree branches should always support you while the lighter green bush-like canopies are meant to feel unsecured and unreliable so it feels like a risk to be in them and encourages you to instead rely on the dark green branches.
In regards to the melee, I do plan to rework it to feel less sluggish later down the road, but the intent to melee is the unique power of destroying some enemy weapons and armor in one hit. (Some enemies aren't staggered if they have armor on) I'm not sure if the demo has it, but melee can also destroy many projectiles. (In the current version at least)
Anyway, thank you very much for playing and leaving me some feedback! As I keep developing the game I'll try to improve it more!