Coupons won't stack with GGC's, because they're both coupons. Unless someone manually overrides the pricing, it's one coupon per item per sale.
RZ certs are considered payment, so they're stackable with GGC's or other coupons.
The 12% coupon wouldn't stack with a GGC, though the $10 item included in the mailing, if it's a RZ cert, will stack with a GGC.