It seems to be taking place immediately after (or days or weeks after) Elturel was returned to the Prime Material Plane - refugees provide evidence of that.
Also, Zevlor was a member of the Riders of Elturel, one of the tieflings who deserted Zariel. I can't imagine him being with the Hellriders in less than a few months or a year.
I don't think he did abandon her. He was in Avernus with the city wasn't he?
Then how come she (Zariel) is still ruling in Avernus, and hunting Karlach? He did abandon her, if we look at canon storyline, where Zariel's army first have some deserters (with a reason of her pretentious crusade to turn out a folly, but still), and then she is trapped there and loses the battle, switching sides and swearing oath of fealty to Asmodeus. So either Zevlor is one of those who deserted and left Avernus with others thus speeding her turncoat action, or he has returned after the city was saved by the unknown adventurers (but Zariel herself remained a fallen one). In that case, he is one of those who remained by Zariel and was saved by those adventurers. No other Hellriders are possible - they all followed Zariel or wouldn't be called as such. Could Zevlor stay away from this campaign which was supposed to be larger-than-life of every paladin and his comrade-at arms gathered for such a crusade? Theoretically yes, but doubtful.
In any case, that story happened in 1494 in the canon module. We have 1492 year in-game. I'm raising that issue for the third time in this thread, seeing no explanation except for the mistake of BG3 writers, who didn't check the canon thoroughly enough in that case.
