It does mean that her reach into the present day would be harder to manage, but maybe that's not an issue?

...Although I just got a mental image of her ending up on, say, Aldis and basically sticking around as a vengeful planeswalker-killing ghost...

Relatively old, but in the capacity that we have some of the characters running around, it really is relative.
He's only really half as old as the Duchess, give or take a few centuries.

The war between planeswalkers on Acharu occured 4219-4223 A.R. After Apocalypse, before Odyssey.

That's certainly within his active period.

I have Vasilias driven from Tyranix on the list at 3686, about 900 years before the present. That sound reasonable?

On Acharu the planeswalker waged a war against a planeswalker. The war caused the fall of the Djontiru Empire (sort of the Roman Empire of the time). The red planeswalker summoned enough actual live goblins that the survivors formed a breeding population (which probably didn't take many. they're goblins). "The conflict between them raged for six years, fought as much with proxy armies of mortal followers as with pitched duels of spell-slinging and counterattacks."

Other than that, I'm open to changing the other details in the current version of the story, if necessary, to make it fit what's know of the Shifter. As long as such retcons were acceptable to the community.

Well... the only way I see the Shifter summoning goblins is as an Invasive species mean to destabilize resources.
It wouldn't be his style to summon them to... actually FIGHT. He also wouldn't be someone to actually command armies, but simply nudge people into starting sh*t with other people, sitting on the sideline with popcorn.

Something would actually have to go fairly wrong for him to get personally involved.

Nudging people into starting their own fights can fit with what happened to the Empire, there aren't a ton of details yet about the fall. The other 'walker can be the sort who summons their own army. The part about getting into direct spell duels with the other planeswalker could be retconned, or something could have gone fairly wrong indeed...

The goblins though. If he were to intentionally introduce them as a disrupting invasive species, how many would he bring? Millions? Thousands? Hundreds? If that was their history and involvement in the Fall of Djontiru they'd be despised or hated by the Aven at least, though tolerated by the Hyrakarr. The question is whether enough would have died in the fighting to whittle down their numbers to the small, struggling tribe that the modern Dzorao are.

I suppose the other possibility would be to say that the red planeswalker who summoned his own critters was just some random guy, and his opponent was the Duchess, who did whatever it is she does...