So if Hero was first to act it is correct to assume that the Big Blind and player B are one and the same?

In this case u need to put in 715 to call a pot of 865. Thems not fantastic odds... However:

Player B's M is just over 5, so he's certainly desperate. So long as you don't figure him for a higher pocket pair you are fine to call.

On the overpair question--
Has the player been tight since he got shortstacked and has only now made a move, or did he just come off a big hit? If you think he's been camping for a solid all-in hand and when he lost his chips he went down with very strong cards/solid play, I would walk away. But if the player has demonstrated even the most mildly loose play when he was dropping his initial chip stack, I would go for it.

Examine your odds against the following potential holdings:
----------------------------
A-low: 7:3 favorite
Two overcards: 5:4 favorite
Underpair: 5:1 favorite

Also you must consider your objectives in the tournament. Are you playing to win or playing to cash? If you only desire a top 3 finish you would be more apt to let this go and get your money in with less risk. There are a lot of smaller stacks out there who are likely to fall off quickly, so biding your time could be a valuable move here. If you are playing to win, taking down player B's chips will put you in second place with ~2250, an M of 15, well back on your way to the green zone (20+).

Considering your own M is below ten, you are running out of time yourself. So if not now, when?

I hope this jumble of thoughts contributes somewhat...