There are always cheap Mexicana fares that are always upgradeable (3 day minimum, no Saturday night stay required). The seat sale fares are not worth it in my opinion. Also remember that a lot of those cheapest fares do not allow stopovers in Canada (if that is a consideration).

Cascading. If I understand correctly, the answer is no. You have to pay attention to the specific fare routing rule, not combining two or more. You can always test this out (the ONLY REAL WAY TO CHECK) by doing a simple routing (so you don't go over your 8 segment limit) and adding the questionable routing. If your fare climbs massively, you know you likely can't do it (although sometimes experienced travel agents can be of help).