I'm super glad you're enjoying the game!
I totally agree that "Undo" feature would be very handy but I just couldn't make it during the 48 hours of the game jam. I'll consider adding it in a later version with more levels maybe :)
As for level 17, yes it is possible to solve it as well as level 18. It's just these last two levels are a bit harder than the previous ones. Would love to hear from you when you solve them!
Thanks you so much for your interest in the game!