1. Yes, but there should be a separate version of the patch files designed specifically for Fates to account for some minor and some major internal file structure changes. I'd ask on GBATemp since the team is probably busy with university right now. Since you have the special version of Fates, Revelations is already on-cart, so I'd definitely refer to GBATemp for some more advice. (Sorry, I don't remember the patching details anymore tbh..).
2. Not sure what you're asking since you said you had the special edition of Fates, so you should already have all 3 paths locally on your cart. Assuming you don't, yes, you can technically patch the digital downloads, but it's quite a bit more complicated.
At least like, a year ago (lol) we didn't plan on using Treehouse's translation to "fill" in untranslated files. It's definitely an option that isn't technically too complicated to implement, lol.
I'm not sure of the technical details of his patch, but from a high-level view I don't think it would be incompatible. His patch shouldn't have to alter any story-related files, moreso the internal game engine/system itself. We also were able to use the patch when one of our own turned Chrom/Lissa/Frederick into playable characters, so it definitely shouldn't be impossible, but may require a little more nuancing to patch.
--Take all of this with a grain of salt, I don't work for the team anymore lol