Reposting my thoughts on an undo button from below:
I think an undo button undermines the point of having a failstate. That being said, level 7 is far too long and punishing without an undo button. I'd like to believe each level can be designed so an undo button isn't needed, but I'm certainly not good enough a designer to pull that off well.
Thanks for checking out the game. I'm glad you like the crabs. The sprites were a lot of fun to make.