Just a few general thoughts
First, I think it depends on how much you paid. I've bought microwaves at all sorts of prices. Some have lasted for yonks, some for just a couple of years. It's generally the magnetron that goes but other bits, including the door and its fixings can go wrong or fail
Your dates are not clear. When you say the door was replaced in September, do you mean 2009?
You do not say what the problem was in November (2010?) but I assume it was again with the door
What I would suggest is that you trawl the internet to see if you can establish if there is an inherent fault with the door on this particular microwave. If you can establish that there is, it may be worth writing to the manufacturer to see if they are prepared to help as a matter of goodwill, either with a contribution to the repair, or an allowance against a new microwave
Off the top of my head without knowing all the facts I'd say the retailer has probably gone as far as he is going to go, notorious or not