As far as ruling out MP because things would have happened "too quickly," keep in mind a couple of things:
1. Mike already has a relationship with Mustaine and Ellefson, so things could indeed fall into place VERY quickly. I tend to think that if Mustaine called Portnoy and said, "hey, our drummer is out. You interested," Mike probably would not hesitate to accept right on the spot.
2. Mike has long had a habit of jumping the gun and being premature about things, so even if it might be too early to have all the details inked, him considering it a done deal in his mind simply based on initial overtures would not be out of character.
Not saying any of this means he IS the guy. But the timing alone isn't enough to rule him out.