I'm not much of a platform player, so take comments with grain of salt.
I like the sparse and plain look, including the stickperson.

--> The instructions need a little bit of tuning. I can see how you're trying to introduce "just in time" instructions, but there's some standards you might assume (like A/D), and you might consider not forcing people to wait (for the spacebar). Especially after the first go through. You might also consider allowing arrow keys with "up" being jump.
--> I find myself sometimes being able to run on falling platforms and sometimes not. Is that intentional?
--> I find "double input" doesn't register (e.g. if I push "d" and "space" at the same time, nothing happens. I expected a forward jump.
--> In levels 1, 2, 3, I find myself sticking to walls sometimes. I don't know why. Is that the "wall merge" mechanic creeping in before introduction? Or maybe a bug?
--> Level 2 is nicely laid out - the going under and then backwards is a nice touch, especially if you are going to have players go back and forth as they go up, etc.
--> I find Level 3 really hard and gave up on it (like I said, not much of a platform player). A simple "it's probably best to keep moving" or "I'd better race through here" would help.
--> Even so, no matter how I race, I can't seem to keep up with the block falling! I also find that I am dying from falling, without really understanding why (I *seem* to fall, but when I hit the block (nto spikes) I sometimes die).
--> I would suggest at such an early level, and while introducing a new mechanic (the falling blocks) that the jump distance (and likely the timing of the falling blocks) be more generous. You want your players to feel like they're successfully learning. To me, at least, this feels punishing.
--> Another generous thing would be: ability to jump off a falling block. I can't seem to do that.

edge-detection is very weird -- I'll get stuck on a platform, unable to walk nor jump, if I'm too close to a corner.

Some of the puzzles are clever and the final one stumped me for a while. But other than that, the game has some serious problem. The physics feel terrible. The combination of near-instant ground acceleration and very limited aerial mobility make precision platforming a pain. I think the biggest problem here, and why the game has such a low rating, is that the game looks sloppy. There's just not any visual appeal in general. The entire area is bland and empty. The contrast is too low, it's almost hard to see the light grey platforms against the white walls. There are issues like the closed and open door sprites not being aligned. Also you set the height of the viewport too small. The height needs to be for the entire frame including the unity bar at the bottom, not just the project itself.

SCREEN TOO BIG
(nice game but at level 16, there can only be one box existing. if i want another box, the previous box evaporates. is this what it's supposed to be? i feel like it's a bug)
(nvm i think i get it now)

Very cool merge platformer. I made it through all the levels in spite of my lack of coordination with the A/D and E keys and spacebar. Some of the levels were quite tricky. If you don't mind the suggestion, I would prefer the controls be changed up to arrow keys for all movement including jump. Then the E merge function wouldn't interfere with the jump mechanism. Also, I needed to reduce my screen size to 80% for good gameplay. Love the dreamy vaporwave music. Overall, this is an excellent solid platformer. Looking forward to more levels of this one! Cheers! ;)