That upscale was made by me, so I'm glad you like it.
To answer some of your questions, I did upscale all the backgrounds last year, which you can find here:
https://www.reddit.com/r/GameUpscale/comments/buizw0/the_curse_of_monkey_island/ (including several comparisons), and those awesome backgrounds seem to lend themselves perfectly for upscaling, without much manual cleanup necessary.
I am hoping to upscale all the game's other cutscenes as well, though unless the ScummVM devs find a way to add support for upscaled cutscenes or cleaned up backgrounds, this is all just a big what-if. Which in turn answers another question: As of now it's not possible to put these upscales back into the game. The way the game reads and displays graphics requires them to be in a specific format (in image size, color depth and compression), so bringing these remastered versions back to something the game can display, puts us just back where we started.
I have several ideas how and where to improve the upscaled intro, especially the parts where the camera closes in on the bumper car, or where it floats away from the camera would benefit from just tracking the full-size frame onto the whole thing. The way it is now, it's clear that the algorithm had to work with very few pixels in the distance.
Audio could use upsampling from 22kHz as well, but it's not something that seems to be possible with current AI tools yet.
