@DandyBoyApples They can be, and are, especially DDLC. Theres a whole mod community out there, lol.
From a speculative standpoint, as I'd like to imagine, Monika is primarily only able to work with in-game assets. It's why she isn't able to fully create her own route without breaking the game. Whenever Monika DOES manipulate the game it's where she is taking already created content and, like you said, glitching it out by (for lack of a better term) Photoshopping realistic eyes/mouth, adding black pixels, etc.
If this sentiment holds true for all of the game, it doesn't point me in the direction that Natsuki has some kind of immunity. It rather points me in the direction that the game, without Monika's interference, has death scenes for Sayori and Yuri included.
Monika can't create content, only screw with it a bit; she can't create a cutscene of Sayori hanging herself or Yuri stabbing herself. Yuri's eye sliding off to the side? Editing black boxes over Natsuki's face? Sure. Easy enough.
But a noose? The image of Sayori being hung? All that? Those already existed in-game since Monika can't create them. All Monika did was speed up the game script to reach those moments. Natsuki, for whatever reason, was never planned to have a death scene in the original base game. It's why her death was edited in crudely, much like all of Monika's edits with Natsuki.
It isn't that she's immune to Monika per say, she just never had a death scene (or anything physically negative) originally for Monika to accelerate to.
1. Nearly impossible. It would just end with him and Monika editing each other back and forth. No progress would be made.
2. No other locations exist in the game. MC has just as much power as Monika does, thus he cannot just create places that don't have in-game assets.
3. What would this accomplish?
4. No other girls exist in the game as far as we know. Much like #2, no assets exist in-game and he cannot create them just as Monika can't.