How different would things would have been for Robb’s strategy if Hoster Tully had been alive and well to participate in Robb’s campaigns?

Good question. I think the major difference is that the Battle of Golden Teeth and the First Battle of Riverrun wouldn’t have happened as in OTL – Hoster’s far too experienced a commander to make the mistakes that Edmure did.

Instead, you would have had something a lot more like the Battle of the Fords but from the other direction, with Riverrun easily resupplied and reinforced from the lands inside the Trident – this probably would have stopped Jaime’s army in its tracks, forcing a stalemate on that front. 

The main question is how Hoster would have organized resistance to Tywin’s army swinging down to the Gold Road to come up from the south – my guess is a more organized combination of sieges and scorched earth tactics, intended to slow down Tywin’s army, force it into multiple assaults, and prevent it from being re-supplied, culminating in a last stand at Harrenhal. 

Which probably would have meant that Robb would be looking to knock out Tywin’s extended army first, as opposed to relieving Riverrun.