Can’t set up a test session at this time, but we have some more details, so let’s see.
I assume that you may not been into Control Room yet. Having experience in studio-recording already, it should come easy.
The Talk Back mic is a function of the Control Room, open up CR editor and assign TB mic there if you haven’t already. VST Connect integrates tightly with the Control Room, so when problems arise, this is your second stop. First is “Create VST Connect”, but this may not solve all issues if you have configured too much yourself, so it counts for an “Empty” project. So, more datails:
“Yes, I see the metering as you describe and his Presonus interface” well then he should hear it. The “Master” channel should also move, as it should receive the input. Master is what should be on Performers’ headphone so if any of these "should"s don’t apply, there’s some serious flaw that we haven’t come across yet, like no master bus or having the direct/daw control on the Interface set all to direct, but then he should hear himself for sure.
So: Cubase Mixer Cue + Talkback → Control Room → VST Connect → Internet → Performer → “Studio” Channel. “Studio” + “Mic” + instr + VSTi → Perfomer “Master” channel. The Master channel also has a bus assignment at its top, I assume you have set that to Presonus Headphone output. Make sure Performer hasn’t messed with Presonus HW mixer.
“Mic” + “instr” + “VSTi” → Internet → Cubase VST Connect → a) track input and → b) VST Connect Monitor plugin in following Control Room Master (or Headphones).
I repeat that when Performer Mic channel meters move (in the VST Connect Performer Monitor mixer), “Master” must move along with it (plus what is beeing received from “Studio”) and thus be heard by Performer if Master output is assigned to his Headphone port.
“I have also done my best to emulate the Cubase setup shown in the primer for the mixer and Studio control modules” - what “primer” are you referring to? Will check that and the manual, as it appears to be outdated. Just use “Create VST Connect” (with an “Empty” project!) and then let us know what doesn’t work. And look at the LEDs, they should normally all be green (occasional exceptions are ok). Hope that helps.