First of all, I've never heard of a 7 year 100,000 mile warranty, but I've never bought extended warranties. Second, if he thought you were rude for not bringing it to his attention sooner he is an asshat, and I would treat him accordingly. My question to you, do you really drive the car under 10,000 miles a year?
BTW, my fundamental answer to your question is that if you signed it knowing that you were signing for 70,000 miles, I would go back and change it. It sounds to me like it really was an honest mistake, but I still would make him pay for his attitude somehow. Drink all the free coffee you can at the dealer while you are there. That is my standard joke when a dealer offers me something to drink....I want everything I can get for free because I know I'm about to pay dearly for it!
2012 BMW 550 M-Sport
2014 Lexus GS350---Wife's car
2013 BMW 1600 GT---2011 Kawasaki Ninja 1000---2012 Ducati Panigale