Nope, that was the only unused item in my inventory. I thought I had tried out all pairwise combinations, but I guess not, as I found the one that got me to the ending! Thank you!
But that didn't make use of the formless objects. I assume there's another ending, so, I'll try to find it…
I tried combining the formless objects (well, formless object #1) with every other object, and tried using it on everything across all the rooms, all to no avail. Any other clue about what I need to do to make use of the objects, or what I should be expecting? Thank you!
One recommendation/request: since there's no save feature, I am doing repeated playthroughs, and the only really annoying point is the bones game. I know the way to always win, but it still takes a while to play through the multiple times. How about speeding it up? E.g., maybe make cut out one round (maybe start with 8 bones, for 3 rounds, and not 25 for 5 rounds? And/or round up from half of 3 to get 2 of the 3 bones, rather than 1 of the three and having another round)