When I used LendingTree, they made it clear that they were simply find companies to give me a loan and that they had no responsibility over those companies.

The quality assurance you speak of is people like you giving them feedback. Telling them about this is the way that LendingTree can decide whether or not keep Ultra as one of the lenders they use.

Go after Ultra, but unless LendingTree promised you 25,000 United miles as a satisfaction guarantee, I just don't see where you go this.

I'll bet, sadly for you, that Ultra will claim that they never gave you a locked in rate. If they did, you'd have something in writing from them.