Alright, the game I mentioned was finished. Now I'll get to playing standalone CSD v28 and VMC v82/Diplo-City States v5/CSD mix each and try to find out whether the problems persist.
For the sake of letting me have complete control over test parameters and making it go along more quickly, I'll enable IGE as well and start the game at a late era, to found the World Congress instantly and spam every CS with my embassies from Great Diplomats. Just adding IGE won't affect what I'm trying to find out, would it?
Also, another kind-of bug-like thing to address: Some of the C4DF messages, which you said you fixed in the v81 DLL, still aren't appearing. Most notably the leader greetings between a master/vassal relationship in the initial diplo screen, they merely show the default messages instead of reflecting the current vassalage relationship. Many of the messages such as the 'what do you think of X' messages and tax rate change comment messages are working properly, so I guess you missed making some of the messages activate? Could you look into that?
EDIT: Wait a minute... looking into the Github I think I found what I was looking for. I downloaded the .zip and I think the usual files are all in there.
Except... the .modinfo file.
This won't work without that file, right? Will copy/pasting the .modinfo from the version you linked to in post #304 work?