The lack of discount combination has nothing to do with combining offers and all to do with when the offers end. Canadian ticket discount ends on May 20. You can go to the Disney website now and book a package for dates before that and you will be shown the Canadian discount ticket offer as the ticket option for the package. If you want to travel after May 20 you have to work the tickets and the room separately. So you still get both, just booked separately.
Now as for FD vs. ticket discount. For me it was a wash. Same price either with 10% room discount/canadian ticket with DP, 10% room discount/canadian ticket OOP or FD. So I went with
free dining because it gives me the most money towards an AP upgrade.