I assume making the ball and the X float is a result of misunderstanding the restrictions... Just by not making the ball roll it would have almost perfectly fit in. It still goes to positions that don't align with the pixel grid, so does the floating X. Same with the particle effects.
Otherwise, there isn't really problems with the level design. Although there are different routes you can take it is a linear platforming challenge. At least by going right at the beginning it was. Linearity is not a problem, but since it isn't made obvious that I'm going the right way, (although there wasn't really another way I could go, since I couldn't even backtrack), it felt like I just stumbled upon the goal.
I personally noticed the R, and got that R respawns me, although I see no reason not to respawn automatically.
The movement didn't feel floaty, so nice job.
Overall pretty good, however, as already mentioned, I cannot rate it high on adherence to restrictions.