I think it's possible that Mihawk simply acceased the Mariejois passage after defeating Krieg. We know Krieg's whole fleet was starving to death, then there could be some considerable time spearating Krieg's downfall at the Grand Line and Mihawk arriving at the Baratie again. And like you said, we don't need to go that far. Mihawk is pretty strong, I can see him fighting against the Calm Bet by himself, be it through Kairoseki or just his sheer strength.
As for Don Krieg, I've no idea. They should have been crushed and remained at the Grand Line. Oda had already planned the Red Line since he mentioned it beforehand, but maybe he wasn't thinking of it as a mountain.