In theory yes, but SpaceX would have to purchase 4G/5G terrestrial spectrum which is extremely expensive or sometimes unavailable at any price.

Yes! Many comments on r/spacex suggest that being a good use case for them but they have one major downside. Weather can affect satellite network connectivity which brings their availability down a few nines and many people expect most everything to run 100% of the time. I know growing up, I convinced my parents to try out directtv and my dad always got so mad when the weather caused the satellite tv to go out so it isn't perfect. Therefore, I don't expect it to be used by most people unless their only other opens are hughesnet or dsl.

Yeah, that's basically feasible and your intuition of the cost-savings / requirements is accurate. A cell phone monopoly doesn't have any incentive to pass on those savings to customers, but maybe a small group could band together and buy a portable base station to share.

Remote towers often have wireless backhauls anyway. But if you only want a cell or two of coverage surrounded by a vast dead zone then it's likely viable somewhere.

No, it's impossible. Antenna theory dictates what the size of an antenna can perform at. Without significantly reducing the noise temperature of the antenna much farther than anything else out there, it will never happen. And noise temperature is pretty much guided by physics at this point.

I'm not a hardware guy, but i doubt it. They would need to reduce the size and power requirements by an order of magnitude each of which we are currently struggling to do nowadays. Most importantly though, the wavelengths they are using i do not think penetrate buildings well and I don't think any of other wavelengths can effectively transmit that much bandwidth through buildings from space efficiently/economically.