Big breakthrough, but it comes with some pretty bad news.
Good: My networking code is solid and more or less done.
Bad: To fix multiplayer desyncs I have to replace the entire physics system. The methods I went about doing physics I thought would be safe, were in fact not, due to the way Unity handles colliders behind the scenes. This means possibly weeks of work (and some paid assets) will have to be replaced and I'll have to set up my own. This will take some time.
More Good: A few performance and custom physics issues I have been getting will be completely swapped out with my own (hopefully lots faster) bitmap collision system. Still need to do some performance testing to see if this is viable or not.