I'm pretty sure Rick wasn't involved in the creation of Avenue Q. He had worked with the composers for that Kermit Prince of Denmark thing they had done previously, so they went to him again to help develop the puppets for AQ; all the early stuff was already conceived of at that point probably.There seems to be a lot of controversy (and I use the term lightly) surrounding Rick Lyon and his involvement with Henson, the Muppets, and SS. The thing we know for certain is that he ended up being let go from SS, and he then went on to create AQ in a double-sided paying homage and take-that jabbing kind of thing; the details of which are kind of inconsistent.
It was Robert Lopez.One of the composers (I don't remember) probably was the one with sour grapes toward SS when they interned there. There's a lot more "take that!" to children's programming in the original demos I've heard that was mostly cut from the final show.