Based on the changes Marriott made to their system last year, my understanding is that if you cancel a reservation that has a cert award attached, the award cert will be automatically redeposited back into your points balance. For an award cert received through a Travel Package you will NOT get the full award cert value. I think the amount is around 35K for a Cat. 6. (I may be off 5K or so as I have never actually returned a Travel Package hotel cert. A search should bring up what the exact amounts are.)

The way around this issue is to create a "dummy" reservation for a hotel and have Marriott attach your award cert to that "dummy" reservation, then cancel your original reservation. Once you decide on what you really want to use the Travel Package hotel cert, you can then call Marriott back and have them attach the award cert to that reservation. You can then cancel your "dummy" reservation.

Its a pain, but it should work.

Yes, you can use a 7 day cert for an 8 day stay. Your stay has to be contiguous nights, but Standard Award availability does not.