Okay, well... good news, it works now! I made a new class library, reserialized from that, and it worked as soon as I updated the references.
Bad news, I still don't know why it wasn't before?
Well, I know why it didn't work last time I tried to use a class library - I made a ".NET Standard" library, which apparently targets really old versions of .NET - like, to the point I didn't even have an mscorlib old enough to be compatible. Cleared up that problem with a NuGet package, but I'm guessing it being the old version messed with the content manager.
I'm still not really clear why it couldn't deserialize from the main WrathOfTheGods .dll, but my money's on incompatibility with the Android version.
@Jjagg I did try the debug mode (it's under the Build menu) and running it from in VS, but neither one let me see the exception. It just said "The program is in a break state, but there's nothing to show since it's all external code." or something similar. Maybe there would've been a way to investigate the exception from there, but I couldn't find it. If you want to investigate further you can check out this commit on Github: https://github.com/littlebeast/WrathOfTheGods/tree/cba65e7f74b17b9f4e7ce447e67108df11af21e9