Not so simple. If Mercedes decide to just control their own pace, there won't be any gap in the midfield to pit and come out to string out a few fast laps, the guy who pits will be right in the thick of it. Nightmare. You need to be ahead of both, and the victory is there for the taking.
Possibly, but that implies the Mercede's pace will be so slow, no gaps will open, thats quite unlikely.
Of course we can have Grosjean simply drive into them at the start.
But well, third place man, if it's Alonso, even at Monaco may get ahead of one at the start, (this would work with Vettel/Kimi/Massa too, but I think them getting to second off the line is less likely), in that case, simply wait for the lead Mercedes to pit, then do a fast lap or two, may work. Or even go for the one stop.