Not to burst your bubble, Bruticus, but no dealer is going to assure you that ANY repair will be covered under warranty until after they inspect the car. Just be sure to ask them to get your permission before doing any work to the car that will cost you money. Loaner cars are at the dealers discretion...you are not driving, nor did you pay, for a Lexus. It's sad but true.
You should never need to give a modern, fuel injected car throttle input when starting. Good luck!