In this buisness we run into the fueding neighbors all the time (I did get the part where you get a long). We all have stories about how whacky it can get.
To get back to your question. Since you have a view easment, or right of way, that proscribes planting tall trees that will disrupt the view, it seems that that should allow you to get acess to determine if these trees are naturally grown or landscape installations.
This may require a court injunction being that you have to enter the property.
I don't know if you have tried this, but since you get along with the fellow, get together with the other neighbors and offer to cover the cost of the removal of the problem trees and the current amenity value of said trees as determined by the average of a panle of three consulting arborists, one or tow of his choosing. Replacement material must be of culivars/species that will not grow to block the veiw in X number of years and can be maintained inperpetuity at the expence of the adjacent property owners with the veiw easements in a maner that will not cause a long term hazard due to decay formations.
(And yes, I'm not a lawyer, you would need one to write any agreement like that up. I have to mention that or smoeone else will
)
One of these options could be coppice, where a tree is cut down to the ground on a regular cycle and allowed to regenerate from the stump.
I find that when dealling with this type of person, you have to leave them feeling that they got the better of the deal.