^ The geometry of the situation (when Ollie pointed out where everyone was) suggested that they were actually pretty close and it was right where Sara turned to look. Certainly not conclusive, but that was the strong impression I got.
Plus, where did this other person go? I suppose they could have run away, but, unless they were in collusion with whoever shot Sara, you'd expect them to show up and explain what they saw.
Apparently there's only one arrow manufacturer in the world of the show. Otherwise they should have known much sooner that it wasn't the same bowman.
I got the impression that most of these bowmen make their own arrows. Whether they have their own style is harder to say. Certainly, Ollie's arrows look pretty uniform and I'd imagine that most others do too (since it's cheaper for the production budget).