Skin effect with 50hz yea, no not much.

Ok so every time you change the voltage level you still need a transformer and a inverter to create ac, so no it doesnt make any sense.

Dc transmission lines at 10kv? Dc transmission lines make sense when you need to pass a really long distance or you can only use underground cables because those have a much higher capacitaance, i.e. high reaktive currents.

If you need to supply your rural area or town you will go from some 20kv lines down to lots of 3kv and then 400v lines with multiple transformers everywhere. Dc transmission doesnt make sense here, but supplying a grid from really far away with a single line? Yea.

Yea have fun transmitting a decent amount of power with 240v over a meaningfull distance. Also most generators produce ac anyways so why would you recitify it at the generator instead of your device after a transformer? You still need all kinds of different voltages everywhere in your electronics and this means you still need to regulate it.

I am not shure how the american wirering worls out but to get from 240 to 120 you still need a transformer… or is it 240v between the different phases and then 120 from phase to neutral?