The tension not relaxing with the foot up is an issue, but I am not sure if it can impact stitch length.
Have you removed the bobbin case, and cleaned under and around the lip of the race, and cleaned thread gunk out of the small slots of the case? Although, this would likely impact stitch quality, not stitch length.
I don't like to take apart tension units until everything else is working like it should. It helps me isolate issues, rather than expanding them.
One thought when oiling, some of the rotating junctions have ... Oil holes on the collar, if that makes sense. These are easy to miss, and the manual doesn't mention it. It does make a difference, but oiling alongside the collar is still a big help.