It all reduces to an important point from Collins; one which could seem counter-intuitive at first, but makes more sense the more you think about it:
"I would calculate that the Labour party is too spineless to do anything serious - the evidence for this sorry conclusion is now overwhelming - and that therefore if [Brown] did make [the Legg investigation] a test of his leadership he would win by default."
In other words, Brown is actually in a position of some security (until the election, that is). Sure, in many regards he's impossibly weak. Sure, he may even face more challenges, of a sort, between now and May. But he can also take risks which would cause difficulties even for stronger leaders – precisely because he knows that his party lacks bottle. Whether or not he starts taking those risks is an entirely different matter...