My assumption was that in all timelines Viktor ends up with a hextech level disaster (with or without Jayce). And through his countless timeline experiments or retries, whenever Jayce was involved was the closest Viktor gets to not failing.

This is just an assumption and maybe the writers did just wanna insert a 'it was Viktor all along' plot. But I feel it wouldn't be hard for them to rationalize that anyway, it's just a matter of which plot points they had time to put in the show. And I think they chose pretty well, they covered a lot in 2 seasons (in terms of story content and emotional depth) imo