Hydroelectric plants along the La Grande river in northern Quebec run a substantial part of New York and Boston. The power is transferred across about a thousand miles. Transmission loss is about 10%. Ten percent would be intolerable for fossil fuel generation but a quite acceptable cost in that context, given the generation is so cheap.
No, just a very high voltage grid since P=I^2*R and with very high voltages I (current) decreases proportionately and therefore exponentially in resistive power loss.
The problem with the current crop of REBCO superconductors is not the cryogenics but the actual ceramic material being brittle. You can't make a wire or a cable, you can only operate with stiff, thin, fragile bars. On top of that. it's not cheap, $100-200 per meter of typical 200kA power band. I suppose copper is like an order of magnitude cheaper just for the cable, ignoring the whole liquid nitrogen piping.
Probably not in China, since the math with respect to losses works out. The distance from, say, a sunny place like Lanzhou to Shanghai, or even to Guangzhou, is relatively close.
But of course, exporting from Australia to the rest of the world will be problematic. Not sure how that will work? My impression, however, was that they were only trying to get the energy to Singapore? Which should work. It is 3 times longer than what the Chinese are trying to do, and underwater. But again, theoretically, it should work.
Australia could make their land more productive with desalinization. Expensive energy is the main reason it isn’t done (what to do with the waste brine is the other bit).