Update, I’ve been updating plugins on the Mac all morning. Not sure if this is what changed things, but the session in question now opens and passes audio just fine. 
TLDR:
If the plugin library on each machine is almost identical, can I move a session from PC to Mac or vice versa and expect it to work identically? As of right now, my answer is “no”.
I moved a session created on a PC using Nuendo 8.3.2 to a Mac using Nuendo 8.3.2. The session opened, I removed the plugins from the session that I don’t have on my Mac. But no sound would pass through the assigned bus.
I bypassed all plugins on the bus, still no sound. I created a new bus, routed everything to that bus, and audio passed just fine.
The failing bus had plug ins on it that were present on both the PC and the Mac.
Another odd thing, I was not able to bypass any plugs on the Mac session at first. Clicking on the channel or individual plugin bypass button had no effect. I THINK routing signal to the new bus reset something correcting this, I can’t be sure. This was a weird one.
If anyone can speak to this workflow, moving from PC to Mac and back again, I’d appreciate hearing about your successes or failures.