It worked! Thank you very much, the game was great :D I'll definitely spread the word about this trick, in case some other devs are willing to try it.
Some curious things I noticed (no problem to speak off): the 20SM.exe I initially downloaded is 153 650 kb whereas both of the current downloads are only 71 354 kb. Did you change it at some point or did I download something weird? I'll note that this new (?) version doesn't work either, but it does manage to enter fullscreen, whereas the former only became a borderless window stuck to the upper left corner.
I also found a minor bug: if I return to the menu after starting (not resuming) a new game, the slot selection maze's dot will not move until I relaunch the game.