Tell them if they do not bother to do what you have told them, you will bring along a journalist, say Next Magazine. Meanwhile, video tape record (by your cell phone or whatever means) everything between you and themselves. Of course they would tell you that you cannot tape record bla, bla, bla. But why not? There is nothing to hide, right? BTW, take a camera and take a photo of the indent, just in case. In fact, I would bring alone a friend and use two cell phone to video the whole scene (serious!!).
If salesman and the manager refused to do anything, request to speak to their senior. The point is, if they "fixed" your problem with soft approach (ie, no exchange is required), they do not have to report the whole matter to their supervisor. So, stand and fight. This is your right and don't you forget that.
Salesman told you that they have already registered the vehicle in you name so on and so forth. But that is their problem. They should have check the vehicle BEFORE any registration. This is their fault! Of course they don't want to do any vehicle exchange - they would have to scrape this vehicle as a used car (I assume that you aware Toyota (HK) have used car for sale).
Remember, stand and fight and tape record the whole thing. They can't treat a customer like that. This is your right.
Nil reply from the sales yet. I went back to the car park again this evening, hoping to take a few photos of the car, but it's already gone. I'll do everything to get what I should get.