Yes, free nights can be canceled. Check the reservation details. Some require 3 days in advance, some just a day in advance.
Yes, you can only book one night at a time when you book online. Or you can call reservations and book consecutive nights. Yes, you can book a night with points that you should be able to convert to NBT/FFN when you've earned it.
Don't expect to be able to book a FFN/NBT night at the front desk. Take care of it before you arrive at the property.