Wow, thats a deep analysis, thanks a lot!! I will try to answer you:
-The game is inspired in escape goat and some other games so you are right.
-I don't know if you are playing the last version, if you don't see a crappy title screen then is the old one, in the new one i reduced the number of objects and collision checks by 2 or 3 times so the game should run smoother. Please try to refresh or delete the cookies of the game and tell me fps again.
-The problem with the jump is because of FPS, so maybe now feels better. About the "jump to the spikes" mechanic, i will try to place the spikes farther.
-The respawn with soul is intended, but now you said it i think that maybe i can remove it, but then i should be more careful when i design the levels.
-LEVEL1: You are right, the idea is to teach that you need to press a button to open the door, but the positioning of the objects is not correct
-LEVEL3: this mechanic can be frustrating in slow machines but i will try to put the spikes farther as i said
-LEVEL5-6 i will switch them, you are totally right in this one.
-LEVEL7: again, the fps problems. At 60fps the level is easy but i will try to make the "third floor" a bit easier. The fireball one is to teach the player that fireballs can kill slimes but well, i can design a whole new level for that.
-LEVEL8: Maybe it's a bit hard to discover how to play it, you are right, i want to maintain the mechanics in the level but i will try to do it more clear.
-LEVEL9-10: this level is a very old one, I will change it soon, it's more a placeholder than anything. Anyway, i never thinked about that situation, that's why i should not test the levels . And level 10 is a boss that i dont like it so much but it's only to see the concept.
-LEVEL SELECT: right now you can select the level you play pressing 1,2,3,4...0. I'm working in a level selection screen but with 10 levels it's fine and i want to do other things first.
-Even you can continue playing after level 10, the levels are not tested so, test them if you want but they will change for sure.
THANKS A LOT for your impressions and constructive criticism
EDIT: I just have an idea, what do you think about if you have a soul out and die, you return to your soul? The game will be a lot less frustrating and the feel of "power" is greater.