Going by the miles listed in the manual, the timing belt shouldn't have been an issue yet, so it must have jumped for a reason, and should be covered despite what the warranty rep says. The fact that it jumped at all would make me suspect a faulty component somewhere, so I'd most likely opt for replacing all associated parts; tensioner, pulleys, and belt. I'm sure there are differing opinions about going with factory or aftermarket parts. Even though it was a factory part that broke prematurely this time, I'd probably still try to err on the safe side and go with factory stuff.

Since it's going to be torn down that far anyway, replacing the water pump at this time would be the more cost effective way to go in the long run, otherwise you'd just have another similar bill for labor when you did decide to have it replaced. I agree that if you went ahead and bought a new one, the mechanics would probably change it for you (while they're in there) for a small additional fee. And given all that, going with a factory pump as opposed to a no-name "equivalent" should also result in more piece of mind that it was done right.