In one of the post-games I watched, they emphasized how Tom Coughlin continues to "coach-up" his guys throughout the season. They said you are playing a lot of young guys and sometimes in positions you aren't familiar with and that the NFL still is a developmental league. Which I think applies here.
So yeah, Williams didn't have enough sense to not try things he's not capable of but I'll go along with putting some blame on Harbaugh too. Even if he's the #2 or #3 guy, you prepare him for that role and you coach him for it (or make sure your assistants do). And as a punt returner, your first mission is "first, do no harm". If you fair-catch every punt, well, that's OK. At least you aren't losing the game. I know Harbaugh is a very aggressive coach and wants his players to "make plays" as they say now. But sometimes you have to be smart. Put your WR with the best hands back there and make good decisions.