This is pretty ambitious for a gamejam, well done on getting so much done!
"For gameplay reasons" made me laugh!
The controls were probably the hardest part for me. I did manage to get to the platform puzzle and work out what to do but my cart rolled off.. then when I went to fall down I landed on it and it was pushed through the ground (never to be seen again!).
... a short time later ..
I played through again and got further up to where I think gravity changed, then I fell and really wasn't sure where to go so started from the bottom, only to have the cart roll off the platform again on me. I may come back to it with more time, would like to see how it ends.
Some pretty cool ideas in here, very hard to nail in 9 days. Congrats on a fun submission. :)