Thanks for the detailed review!
I was aware of most of the problems you mentioned, a lot of them exist because I ran out of time to make them better. I wanted to make lava more of a threat but I couldn't think of a good way to achieve that. I could punish the player somehow when they touched it, for example taking away the ability to jump for a few seconds but I thought that would be too annoying. Maybe with an easy tutorial level the player could learn the movement and get comfortable with the controls.
Your point about the jump implementation is absolutely right, it should set the upward velocity, not just add a force.
Balancing is also a thing I struggled with. I didn't have enough time to make it right so much of it was just guessing. I'm glad you think it was all right in the end.
So to sum it up, if I could change anything it would be to make the basic enemies damage less in the beginning and implement some mechanic that makes the lava more dangerous.
Out of curiosity, how many seconds did you survive? I was worried I made the beginning too slow so some players just give up before seeing all the enemy types. Also, how many powerups did you get? Their spawn rate could be a little higher I think.