For the moment I am still considering Sirius and Crabbe...

I'm not entirely sure I believe Sirius's claim that his job is done. As previously stated, he could simply be trying to get the attention away from himself.

As for Crabbe, it wouldn't be too incredibly hard to alter the quote to look like a role PM. We've all seen them and know the format for them, so faking one wouldn't be too hard... It, coupled with his vote against Ron, might be trying to get us to go after a townie or he could actually know something about Ron's role that we don't...

At this moment, I am leaning a bit more towards Crabbe... though Sirius seems just as eager to defend his roleclaim...