To my knowledge, you are correct - the northern-most native
Waxmyrtles [i.e. Myrica (morella) cerifera
] occur in coastal southern New Jersey.
Here are some photos of Waxmyrtle in southern Delaware. I believe I took these in February so you can see they are retaining their foliage. I don't know the origin of these (if they are from wild Delaware "stock" or not).
However, previously I bought some "Dwarf Waxmyrtles" (Myrica cerifera pumila) from a deep south nursery and planted them at my house in northern Delaware and their foliage got scorched in winter. So I am guessing northern provenance could be important if you want Waxmyrtles to stay evergreen.
On the Delaware coast (as with NJ, MD, VA, and NC), we have both Waxmyrtle and Bayberry. In summer (when both species are green), to me, they look alike. I took this one on the Delaware coast in summer - but which is it?:
To further complicate things, I have this one (below) in my backyard in northern Delaware. It was from a small plant I yanked up at the Delaware coast. It stays semi-evergreen
- which, around here
, could be either a Waxmyrtle or a Bayberry. Yikes! I guess I need to "key" it out some day to find out what I really have! Or could I have a hybrid?
Close-up of foliage (of my plant):
On the question of Myrica (Morella) heterophylla, I believe that species is also native from NJ south. I used to have one at my old house (also in Wilmington, DE) and it stayed evergreen. It might be one to try for you because I think it is more reliably evergreen (mine came from a nursery in SC).
So I'm not sure if I helped you or further confused you?