How risky is it to return a rooted android device under 'warranty'
I'm not sure it would be legal for them to discriminate on warranty just because a phone is rooted. However, there are some "void your warranty" things you could do when rooted (such as overclock way high, etc). You might find a tool on XDA to restore an original ROM, I did once for a bricked phone. They may have such a tool in-store too.