Or people should just learn to use normal paper size: A4

There they fit because it has one more line in the Class Features field. And as I said earlier, that is the one I use so I see no problems there.... But in all seriousness, it only goes over on lvl 20 with one line, so it is not that big an issue. Or better yet, I'll just give the Letter version one more line and remove one line of proficiencies. 9 should be enough of those I figure
As for multiclassing monks... Everybody should just know that it is a bad idea, it wasn't disallowed in previous editions for no reason! This sheet is multiclass-friendly, but not multiclass-foolproof. I don't see it as a problem that people who play multiclass monks (or any class really) will have to work a little bit harder to make everything fit.
I count four lines that would be needed to make everything pretty and adhere to the style. That is quite a lot of space to save with evasion and quivering palm alone (which isn't even on the second page...), so I'm not sure you can produce that much witchcraft, but I'm intrigued. Why would we want to make things on the third page shorter/less clear? Or do you suggest that just so that everything would adhere to the same style?
I really had to look for what was miscalculated with the Ki. Turns out you didn't mean Ki but Disciple of the Elements. Guess I misread it as saying "You learn two additional..." instead of "You learn one additional...", good catch! From 'all' these 'calculation' things you already found two mistakes after you've looked at half the classes. I hardly think that means all of them are wrong and you should check all that first. But please pick any work-order you are most comfortable with!
Even if you can swap out "Elemental Attunement" for another, you will only be able to do that once you reach monk level 6. So I don't see a problem with automatically adding it to the third page. You can always manually delete it and the action it adds, and add a new Elemental Discipline.
Also, and I'm very sorry, but I find it hard not to take your tone as condescending when you call it a "wretched mess" and things like that. I know you are working hard to make everything even better, and I appreciate it a lot, but I'm not calling your work inadequate in this tone either. I had to check every single alteration you made for whether or not it was able to fit in the allotted space (and a lot wasn't, but that's okay. I had to check/rewrite all the stuff people have sent me, which I think is a good thing because that way a singular 'style' is maintained). It might help if you keep in mind that English is my second language. Or maybe I should just write everything in Dutch or German and have you check it for errors then
On another note, I have decided to use your suggested capitalization for spells, hd, and hp. As feature names were already capitalized I figure spells shouldn't be treated differently. I do want to keep the class names lower case, though. These are used in the descriptions of the same class, so capitalizing them wouldn't really add to readability I think.