As Baze said if you dont fly the AUS-IAH flight everything else will be CXed on you.

However on the return as long as you dont have any checked bags, you can walk away @ IAH and fly to AUS.You wont earn the miles for the AUS flight , just dont make a habit of doing it and you will stay beneath UAs radar

What you need to do is a find if theres a cheaper 1 way IAH-GRR-XXX purchase that along with a 1 way return GRR-IAH-AUS. Again Do Not do this if you need to check any bags!