Great Duck
Well there was a real short-lived Inner Mongolia People's Republic after World War 2. I imagine it would want to reunite with Outer Mongolia but would be independent for a while. Also creating a state, however short-lived, solves a number of practical problems. Also it would be difficult to say who the ministers of either Inner Mongolia or Uighurstan would be since both are actively suppressed and the real leaders are probably underground. But at least don't throw away the tags yet. There are several other Chinese tags which might simply be the ones chosen for miscellaneous regimes anywhere on the planet.
Conversion?? That would be extremely difficult but not impossible. What I mean is that if I save a game in say 1965 or 1970, it should be possible to extract information from the savegame to construct inc files. Then you'd have to convert, say a level 7 destroyer to level 2, etc .... and then determine the appropriate technological level for everyone. Someone will figure out a way to either automate the process with a script or at least fugure out how to use spreadsheets to do the conversion. I've heard that some people are already convering saves from other Paradox games to make resulting HOI2 scenarios.
So my suggestions are: don't use the MEN tag and reserve a few tags for people to put in miscellaneous countries. If nothing else suits, I think all the obsolete Chinese tags -- MAN, MEN, CGX, CYN, CSX, CXB, U87 (Wang Jingwei China) and U74 (Northern China) could be 8 "off-limits" tags reservable for alternate history cold wars.
Also what tag is this mod going to use for Germany? GER or DFR? In my opinion what really happened when the two Germanys reunited was that East Germany was absorbed into (annexed by) West Germany. It certainly was not a reincarnation of the Nazi regime. So I'd also vote for using DFR for Germany and reserving GER for an alternate right-wing (doesn't necessarily mean nazi) Germany. Otherwise reserve the UPR (Prussia) tag.