The cell antennas are omnidirectional but the satellites are phased array. They aren't going to be that narrow, but they can avoid, for example, a city that already has extensive coverage. Remember, these are meant for out in remote areas or in areas of cell outtages, not for downtown in a major city.
What I'm saying is that if it really does interfere with LTE in a significant manner, then T-Mobile, a carrier partnering with Starlink, has a large incentive to work with them to reduce that interference.
I'm not sure what you are getting at. Each carrier would have to make a deal with SpaceX. Doesn't matter if they share the same tower. If SpaceX is interfering with their frequency band, they are going to file a complaint with the FCC
