Defective tire stem - is my tire ok?

So 1 month ago, I got new tires and wheels (BBS RK 17x9 with 275 Kumho on all 4). When I pulled out of the garage on Saturday, the car felt sluggish and the rear left tire looked a little low. I had to run some errands and figured I could fill it up at the gas station by the grocery store. Well by the time I got there (7 miles later) the tire was totally flat. The car wasn't pulling in any direction... it just felt slower. I felt like an idiot for riding on a flat tire

Anyway, to make a long story short, the valve stem was defective. Not the core... but the air was escaping around the stem. It was pretty fast for a slow leak... Lost about 10lbs of pressure every 5 miles. Luckily, the rim wasn't damaged at all. Just a bunch of rubber where the tire was riding on the rim, but it wiped off easily.

I had never seen this before and neither did the mechanic. Is this common?

Do you think I damaged the tire by driving on it nearly flat for 7 miles?

Would the mechanic have to dismount the tire to replace the defective stem? If so... I figured he would have checked out the tire from the inside to make sure there was no damage.

What do you think?

Thanks,
Jim

Thanks for the replies. They were all new rims tires and valve stems. All new.

I can't really see any visable damage on the outside. I'll drop by the shop this week and see what they can do about it.

Thanks,
Jim