I can see a few holes in there though. Possessing someone doesn't change their physical appearance, which is something Abeloth does, possession I mean. That he's old just means he aged. That he is very powerful can be attributed to Abeloth.
I'd say the biggest thing is that SW has proven before they have no issue with using bits of extended universe and completely ignoring others. Example, Chiss being involved with the sith at this point in history, Rakata being involved at all, Sith purebloods existing in this time period etc etc. Bioware is infamous for taking things it likes, and ignoring others, just like LucasArts.
I'm not gonna discount the emperor being Abeloth. I wouldn't also believe anything regarding the emperor's indentity unless it's a confirmed legit Lucas Arts/Disney or Bioware resource. Not extended universe.
The Emperor is not simply possessing people. He is filling them with his essence. They do not change, they hollow out and are instead filled with the Emperor's power, allowing him to act as he sees fit. Abeloth didn't possess people so much as warp their minds and drive them insane. Abeloth was more of a mimic, taking on the forms of whoever and whatever she wished until her true form had been revealed. The Emperor simply fills a person with his essence and allows them to become his Voice.
As for the Chiss, they were never truly fleshed out until Timothy Zahn created them, and we still know next to nothing of their own history, hierarchy, planets, and people. We have yet to truly see anything written or told from a Chiss standpoint which really means that they could have been involved witb the Empire at this point in time, we simply didn't know. Records get lost, and destroyed, and for all we know, the Republic could have had records on file of the Chiss from this war that got destroyed or corrupted in some other conflict. Since the Empire holds all the records regarding the Chiss species, it's plausible that those records were lost when the Empire fell.
The Sith Purebloods could exist at this point in time as well. The original Tales of the Jedi didn't go into the depth of the counter attack against the Sith that the Republic unleashed. Instead, it ended when Sadow led a failed attack against Onderon and was repelled back into Sith space where he was destroyed by his rival. From what I remember, they expanded on the Great Hyperspace War quite a bit in the histories, where the Tales story was actually rather short. Not to mention that Korriban was not the only planet that the Sith inhabited, their Empire was very large, so who's to say that some didn't survive somewhere else? The Lost Tribe of the Sith on Kesh confirms this, though those Sith were all wiped out by Humans.