Uhm that's not how it works. US can enforce a regulation saying that any aircraft entering US airspace cannot have laptops in the cabin and if an airline wants to fly to US they would have to comply with that regulation. FFA can easily impose fees and bans for airlines which don't comply.
Basically US has full control over their own airspace and can control what enters it - just like EU does with their own airspace. US cannot order EU airlines to do something, just like EU cannot order US airlines - but both can enforce restrictions on what enters their own airspaces.
Sure, but the EU or any country can make these terms unenforceable in their airports. Sign a law stating that circumventing such a ban cannot be punished for example.
You can also play a negotiation game. Ban something ridiculous in return. Then say "we lift our ban if you lift your ban, and reduce import tariffs on X by 2%". Then the other party says "we'll reduce it by 1%, and you agree to this line in this treaty, and we'll only ban laptops over 1.5 kg"... and you have a deal. That's how bilateral negotiations go. And I think many commenters believe that this laptop ban has at least partially economic motivations.
I wish the EU had more balls and self confidence when it came to such negotiations. This is very tangential, but I think Germany in the EU, the EU in the western world, and the west (North America+Europe) in the rest of the world don't realize how well off they are, and how much power they still have.
Basically US has full control over their own airspace and can control what enters it - just like EU does with their own airspace. US cannot order EU airlines to do something, just like EU cannot order US airlines - but both can enforce restrictions on what enters their own airspaces.