Totally agree on the undo button; I already have it working on a newer build and will upload it post jam. Was a bit worried it'd kill the flow or difficulty of the puzzles but it's actually quite the opposite, since it removes the need on longer levels to re-do the whole sequence (tho sometimes you need to, as specific moves at the beginning might make the state of the board unsolvable later!).
Either way, thanks a lot for the feedback!