After a few hours of reverse-engineering I can confirm that this game is just unfinished. I poked around in the code and found the check for the M- unlock, which specifically checks if the amount of spent points goes from 1 to 0. Without cheats, this is only possible through M-, or another button which has no unlock.
And the "go backwards" one is not as simple as multiply by 0 exactly. Something like 1 * 0 = seems like it should work, but behind the scenes there's a tracker called the "roll" (a list of numbers) which gets set and reset at strange times. What I can figure is that generally, pressing a green button (notably ++, --, and =) will add a value to the roll, and pressing most other buttons will clear it. The roll is what actually gets checked for some checklist items like this one. So to get this one you have to do * 0 ++ = so you queue the "*0" operation which clears the roll, THEN press ++ to get 1 and put it on the roll, and finally = to apply the queued operation to get 0 and put it on the roll, leaving [1, 0] on the roll and passing the check. I can't tell if this is the intentional way to get --, but it is heavily obfuscated if so.
It was a fun game though, and I hope we may get to see some improvements!