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Who put those wheels on?
They are rusted where they lug nuts didn't cover them. Those exposed threads having a very slight amount of surface rust means nothing.
Discount Tire over tightened your lugs. They went beyond the elastic range of steel and the lug yielded. There is no possible way to remove them without breaking the now work hardened brittle steel. They owe you new lugs, but I wouldn't let them touch it given their poor work to date. Even the ones that didn't snap are still probably in the yield range and will eventually fail once they are "worked" some more.
Tire Discounters (not discount tires) did the same shit on a Fusion I bought used. The dealer had TD replace the tires before it went on the lot. When I went to pull a wheel, I had to lean on the 4-way with everything I had, the metal was popping an tinking the whole time. I got lucky and they all came off. ******* idiots.
Edit: I bet if you get a good pic of one the the still "good" lugs removed, you can easily see the variance in the thread pitch where the lug yielded and got longer.