Over all, I had a lot of fun playing through the level/levels
Section 15 is my favorite, it's fun to play, works out really nicely.
Section 11 was the most difficult to figure out, it took me several tries to realize what you are supposed to do. (probably that's just me lining the tanks up in a way you can get through them and not making the choice to make it easy ^^)
Section 14 could have been a bit shorter, as it felt a bit repetitive and therefore tedious, even though I like the idea and it was fun and relaxing.
Section 17 has a short time limit. 600s sounds like a lot in the first moment, but the try everything green wall ice maze takes up quite some time as well as the path starting with the monster area is quite long. After you know the paths it's relativity short, but for the first play experience I would probably recommend 900 as a time limit.
The overall concept of simulated checkpoints is something I played around with as well, but never made it in any levels, as there are certain problems with it, as if you go for a strict order in levels, the checkpoints have to be mandatory as well, otherwise the player would have to choose the next level from the selection which probably isn't the best idea, therefore a big level looses a bit of the feeling of being a huge level - part 17 here is a nice touch in this regard, even though not without the downside of being way longer than the rest of the sections. The second part is loosing all your items by using a checkpoint, which separates all parts a bit more from each other (in my experiments I gave the player the items after spawn (which was on the tile of the checkpoint, which itself didn't exist anymore) for this reason).
The execution of the concept in cc2 worlds is really well done