Assuming the fence has shared ownership between the owners of the properties it separates I'd say as long as you remain on the fence and only touch property A and D then it is not trespassing as you have shared ownership/permission for 50% of the fence so it's your right to use the fence as you wish as long as there is no damage or you take responsibility for and repair that damage. the airspace exclusion should only apply for extended assisted travel in an aircraft or use of a drone. Honestly seems trivial to me, much more trivial even than retrieving an item a la a ball accidentally thrown too far.

But Ianal..