hmp is a naughty thing, in the end of hmp vertex list, you can find separately the shared chunk border vertices, in the same order as standard vertices i.e. starting from the upper left corner, so it could cause problems. some time ago I wrote a function to access them for proper terrain mesh deformation, but not really used it because later I fixed my problems by a DX function, but if you need I can try to find it (hopefully not deleted).
I would be very happy if you could make it also for terrain, I have some simple game idea where I could use your navmesh+crowd system.