1- Yes, the renewal discount currently is right at $100 for a "Tourist" (regular) AP.
The renewal discount currently is over $120 for the PREMIUM AP.
But, it is often more advantageous (in the long run) to NOT renew an AP if it will not actually be used within 2-3 months of the renewal date,
but instead, just wait and buy a completely NEW AP upon your next return to WDW.
(Do the math, looking that the NEW AP would be good until a year from that new date.
2- Yes, you can add in the value of ONE other ticket (any number of days on the ticket) to lower the cost of one new AP (or renewal of one AP.)
No combining of multiple tickets in upgrades is allowed.