Hey, sorry for the late response.
About the suit progression:
Ever since I added the broken helmet upgrade I've actually been thinking about something similar to what you said.
For example: finding parts of the suit until you finally complete the whole thing and put it on or something like that.
In an ideal world, I'd like to have this system in the game as far as suit progression goes, that's why it's on the to-do list, but I can't promise anything, all I can say is I want the game to begin like that at some point.
I assume the hitbox issues you mean are when you aim straight up near a wall and you hit the wall, if so, then it is on the list, otherwise I'm not sure what issues you mean.
As for your questions:
1. I have most things planned out, every room on the map, item locations, bosses, etc, but I still go by the "get this section done" way as you said it in terms of actually adding things though, since a lot of what I already have planned change over time.
As for how far in development the game is, can't really say exactly but there is A LOT left to do, I'm still considering adding more areas as well.
2. I have no idea how long the game will be, this will vary from person to person.
I don't know what else to say here other than that the demo is only the beginning of the game. (and it may become longer)
3. It will be on Steam at some point, don't know when but it will not be in Early Access.
I'd rather not discuss pricing at this point as the main focus right now would be actually finishing the game.
Hope that answers some of your questions, I'm glad you like the game and thanks for taking the time to give me some feedback. :)