1) You can convert UA or AA miles to Marriott Rewards through Diners Club, you would get .6 MR points for each mile ... not a great rate, but if you really do have tons of miles on either airline and need the points more, its doable. If you don't have a Diners Club card and decide to get one to do this, I'd be more than happy to refer you (so that I get a nice little bonus

).
2) You can make the reservation without having the certificate and request the certificate before your trip.