You're right it won't. The short answer is that adding velocity together is an approximation that only works for low speeds. It doesn't work when you get near the speed of light.

This link explains better than I can https://galileo.phys.virginia.edu/classes/252/adding_vels.html

Imma say no.

Helicopter works by pushing air down. That air hits the floor of the plane, giving the same "weight" as the helicopter just sitting there anyway.

A real interesting question is what happens if I have a big train going half the speed of light. Then stick another train on top going half the speed of light. Does the second train go at the speed of light?

It probably would. Which would you choose?

You'll never convince everyone in the world to agree to ditch their own native language in favour of another.

That's to say nothing of the cultural aspects that would be lost in the name of making things "easier".

That may be a part of it, but it's definitely mainly safety. Even countries where people have always done their own fuel have the rule.

Car engines are hot and are obvious ignition sources. During normal operation the fuel is nicely contained and that's fine. When refuelling the fuel is not nicely contained and can spill. This is an explosion risk.

Its a very small risk admittedly, but still present.

I'm not aware of any incidents at fuelling stations (though am sure there are some) but am very aware of industrial accidents where trucks have left their engine on around hazardous materials and caused an explosion. I'm sure there was a very serious one in Texas in early 2000s.