I don't know if her unknowingly having the serum on her was true for the previous time loop. At least, it seems more poignant that the change in the timeline came from Coulson sacrificing his health to sneak the serum into her gauntlet, as opposed to her just not noticing it in the previous loop.
Fitz and May going to back up Mack also probably played some role in the change, given their prophetic knowledge that Mack and the girl's mother didn't make it to the Lighthouse in the loop and this may have been the event.
If they do want to bring Graviton back, yeah I'm sure you could easily write that the Gravitonium in his dead body still contains Hall's consciousness and everyone else (maybe even Talbot, who knows) and could be used by someone else. But who knows, they now have to go back to the drawing board to work up stories now that their renewal was an eleventh hour surprise.