Hmm, what do you think about having "WAS" added to the 12th blank? https://imgur.com/a/AYQQM4A Or I could just do "W" perhaps. (You interpreted "Go Back" correctly, I was just telling players to go back and solve 1 - 11 properly upon reaching 12. I think I'll move that message to the top of the page.)
Another thing I might do is change the Ink Ring and/or the end of the message -- I purposely hid the U to set up NOW __OY as a red herring (since it seems like potential words), which I'm now realizing was just pointlessly mean.
I'm flattered puzzle 11 was reminiscent of the treehouse -- that area is one of my favorites.