Linux Gaming

I can’t access each other OS’s partitions while I’m on one or the other, correct?

Not entirely correct. Linux can read and write to NTFS partitions. However, probably a better idea, if you have three disks or partitions is to format a 'data' drive that Windows and Linux can share. The exFAT format might be preferred for this.

Make a backup first!!

But yeah should be otherwise copy-paste.

The exceptions are things like ENB mods, but textures/models/quest lines should all be fine.

Definitely keep the original files intact/backup the files before doing anything, but theoretically, I think it should be possible. Likely, though, depending on the game (especially if it's a game not made to be modded) there may be specific things like DLLs that look for a niche Windows component or driver that Proton can't translate and won't work.

That being said, Proton is open-source so there are old versions and forks that may work better, GE-Proton most famously. Also, if the game has built-in mod support or rely on a platform-agnostic runtime like Minecraft Java edition, those probably won't be an issue anyway as the engine/language runtime should handle low-level stuff like that by itself

I think this should work assuming that the game and mods are compatible with proton. Also, you don't need an external drive, you can access the windows partition from linux (probably not the other way around though since linux understands NTFS but windows doesn't understand ext4). If there are any issues, maybe try the "Verify Game Files" feature of steam. This might remove the mods but at least you don't have to download the game again.

First, which game are you talking about? Does it need proton on linux, or does it run natively?

Definitely first install the game through steam on linux and then overwrite stuff.

You should be able to access your windows drive through linux. I would expect it to show up in your file manager automatically and ask for your linux root password when you try to access it.