I've read a few of the posts to your thread and don't know a solution.
You MIGHT do better by writing, rather than phoning, the parties you
have complained to. Send certified mail. "Hazardous trees" are a code
violation where I live, so your city likely has some similar ordinance.
If so, then they might be more inclined to determine who the property
owner is or who has jurisdiction, or decide that someone has an easement
to take care of the trees.