Drones can reach much more than 1000ft, but in the US are not allowed to - 400ft flight ceiling.

Ironically, one exception is flying within 400ft of a structure, as you can then go up to the top of the structure and 400ft above that point.

That's not true, commercial and municipal drones operate above 400ft all the time. Non-commercial/civilian drones piloted by someone without an FAA Part 107 certification and a waiver cannot fly above 400ft. Also, you are allowed to fly 400ft -above- a vertical structure (like a tower) up to 1000ft tall to avoid exactly this type of collision as long as you are within 400ft horizontally of the structure. EDIT: crud, I did not read your post carefully enough, sorry, you addressed this point exactly. My bad.