Nice one.
One thing that is a littel broken: When the signs change and the converbelt backs up. If you have a couple of items that are already off the belt, they don't back up far enough to go back onto the seesaw belt. So you get punished for having placed previously correct items.
Maybe you could have a hitbox, that hit as soon as the item leaves the seesaw? And if the item matches the row at that time, you could tag it with the row, and ignore the negative effects if an item goes through the same row level as the tag? e.g. https://pastebin.com/zi55dgzD
Yeah, that's a big design flaw I wasn't sure how to fix. Marking items as forgiveable by checking them early is a good idea. It sure is better than outright deleting them or telling the player "tough tits".
Alas, 1 week dev time, but this is definitely going into the ToDo, for when I have the time, thanks for the feedback!