If those are all part of the same upgrade path they're still one license.

As for latency, I don't think Sonar should introduce any more than VEPro unless you start slapping on some LP EQs or something. I'm assuming you're using the HDSPEs in ASIO mode and connecting digitally? That should yield really good performance.

For VE Pro you don't need all DAWs open.  They are a stand alone app and you don;t need Midi Over Lan.  You should find people at vi control forum who do this quite a bit to assist you.

Why do you need multiple DAWs? Could you just use a lightweight VST host for the slaves, treating them just as if they were external hardware synths?

I understand all of this.... yes I only need a lightweight VST host, that's essentially what VEPro is but VEPro is not working for me. (See this post: http://vi-control.net/community/threads/vepro-times-out-after-25-hours.60112/)
 
I do need midioverlan because I don't run VEPro in server mode.  My thought was just... I have Sonar, I know Sonar well, it would be a breeze to route everything properly in Sonar, why wouldn't I just use Sonar instead of picking up antother VST host that I'd have to learn how to use and may have other unknown issues.  Waiting to hear from support about the licensing issue.

Yup, it's always easier to use the tools you already know!
 
That said, don't write the VST host route off for fear it's overly complicated. It took me about 2 minutes to figure out how to host a VI inside Cantabile. Another 30 minutes and I was running multiple instruments and effects without touching the computer, controlling everything from my MIDI controller. And Cantabile is not the simplest host out there; SAVIHost has a virtually zero learning curve.