yeaaa NO. thats not how time works even if it flows slower or faster in certain areas since it flows in the same direction. The runecarver couldn't have traveled back in time just because he was put in maw and locked in the tower.
1) Lady vajsh and Draka mentions meeting primus
2) The runecarver made frostmourne and the helm of domination
= the Runecarver can't be the primus. I see the same clues that you see that makes it look very obvious that the primus is the runecarver but if he does end up being that, blizz just shows how bad they are at writing a story that they can put a plot hole in brand new linear content.