High-voltage direct current connections have a loss in the order of 3.5%/1000km (that's approximately 10% loss per 3000km). China is operating several power lines at even higher voltages because that makes it more efficient. The loss for direct current depends on the square of the current, so you really want to reduce the current (which transmitting the same power at a higher voltage does).

https://en.wikipedia.org/wiki/High-voltage_direct_current#Ad...

https://en.wikipedia.org/wiki/Ultra-high-voltage_electricity...

Yes it can. It's a good idea but the main downside is it ignores self-defense risk.

No, there is massive loss from transmission as a function of distance