I have an issue with this quest line. My issue is that I get to the location and Hadvar is not there all I find are two soldiers and a quest marker. Even if I complete the attack on the wagon I have no body there to report too. I am not sure if going onto the fort and taking it out would actually do any good. I have had this issue multiple times with Skyrim on PS4.
I assume you've tried fast-traveling back and forth from nearby the wagon? I suppose you could try going to Riverwood and seeing if Hadvar is hanging out inside the tavern. Waiting close by for several hours might also help.