I didn't mention unseen trunks.
Cell is the first known cause of a time split, he made the split by going to the past and burrowing, to reappear when the Androids did.
This cell, the time traveling cell, did not show up in Future Trunk's timeline's past when the androids did. Therefore, we can assume Future Trunk's timeline was never affected by Time Traveling Cell's arrival, since he never hatched there.
This means Trunk's timeline is the only directly unaffected by Trunk's time machine, and therefore the "true" timeline. >>148012464
I'm not sure I quite get what you mean. The perfect cell of the show did not come from the oldest timeline, because he mentions a past where Future Trunks appeared and kills frieza.