Asia once had a set of two leg series where due to withdrawals after the draw the loser with the best goal differential advanced. If Belize advances over Montserrat but is then prohibited by FIFA from playing in the First Group Stage (Second Round), should CONCACAF invite a First Round loser from one of the other four pairings to the First Group Stage or have one of the groups have 3 teams instead of 4?
Montserrat should advance since they'd be the aggrieved party after losing to a suspended team. Can Montserrat compete at the next level? Not a chance, but perhaps playing at that level might convince a few other players that perhaps have eligibility to play for Montserrat to come out of the woodwork.
I would say just have a 3 team group. The 2nd round group stage is supposed to start on September 2 which is only a couple of weeks after Belize might have to go on suspension again. Just set up the group like Belize will be there and if they get suspended just remove them. That way you don't have to worry about sticking another team in there with short notice. Only the winners from these groups advance so no worries about 2nd place comparisons to other groups.
The Second Round (First Group Stage) starts in September. Mexico starts in the Semifinals (Third Round/Second Group Stage) that starts in June 2012.
Belize is good to go. http://www.washingtonpost.com/sports/dcunited/fifa-clears-belize-to-continue-in-wcup-qualifying-after-government-soccer-body-make-peace/2011/08/17/gIQADnDxKJ_story.html