I don't buy that, really. When you're down as many points as Arsenal was, you can't afford to not care about any of the remaining points on the line, while giving yourself away even more into others' hands. From what I heard back then, Bellerin/Koscielny just weren't realistically available to play, the mere lack of options just had Koscielny still on the bench.
Exactly, there is no way a manager sits their best defender on the bench if it was a match they took with high regard.
As Edonator wisely point out - there was no way Olympiakos was going to beat Bayern at Allianz. So basically all Arsenal needed to do was beat Zagreb at home and Olympiakos by a scoreline greater 1-0, which they both completed without breaking a sweat.