Please be aware that Edge Mode is not compatible with V-Ray proxies.If you wish to reduce file sizes and use Edge Mode it is recommended to use XRef objects instead.
Just for info, for Edge mode, an element is any group of adjacent faces with the same material ID (not exactly as Max mesh elements).
And the collision point is taken at the vertex with the lowest Z in the element. Basically, the best approach for grass, flowers but not trees.
Please feel free to send us your scene to email@example.com
to check it here (in case of big file size, please use a web sharing service as WeTransfer.