Reportedly they changed the requirement for bootloader unlocking to be much harder, with a global limited number activation per day. Basically next to impossible with the official tools.
It won't be long before Custom Rom maintainer will lose interest on new Xiaomi phones.
https://github.com/zenfyrdev/bootloader-unlock-wall-of-shame/blob/main/brands/xiaomi/README.md
Not really.
Assuming you are alone in a car, you will be easily at 6-8L/100km, while flying in economy class you will be around 2-3L/100km per person, depending on how much they sardine can you. Airplanes are actually incredibly efficient, we simply travel much further with them.