The dealer told me the run flats could not be repaired, and new ones cost more than standard tires ($400-$600 depending on size, brand, etc),
is it true that they cannot be repaired ?

I got a third row seat, but I also ordered a separate regular spare, just in case.
I put it in a BMW tire cover and use it for long trips only. Texas is too big, and a BMW service tow truck can take hours to get to some places.