Can I use a UA discount coupon (e.g. %off coupon or zone fare coupon) and then pay for the flight using the UA travel credit coupons I received for volunteering to give up my seat last Saturday morning?

All of these things say "not combinable", but shouldn't the travel credit be effectively as good as cash on UA metal, applied after any discount I can negotiate with other coupons?

Also, my reading of the rules on the travel credit vouchers are that they won't be any good on a code share interisland flight, used with a mainland trip (SFO-UA-HNL-AQ-LIH, for example). Is that correct? I guess I have to break out the interisland flight and use a coupon in that case.

Thanks,
-David