I've narrowed down the problem to be something to do with the way the OP links the files. If i install a new D3 copy in each VM i don't have the issue. I think its something to do with file locking maybe.. I'm not sure exactly which bit is causing the issue yet but i'm getting closer.
I think rather than doing all the linking with a .bat file i might just install D3 on the first VM and make linked clones from that. Sure my other copies will grow a little bit with each patch - but the complexity is greatly reduced.
Edit: redid everything from scratch without linking, can load 10 currently with less than 20% CPU load (hex core intel), there are even still some cores on the host not being used

. Also found better performance to give each VM 1 CPU with 2 cores, 2GB RAM. My PC has 64GB RAM so i stuck the main VM that had everything else cloned off on a 20GB RAM drive.