I'm sure this scenario has been raised before but I'm new to Vmix and I'm trying to find a solution before my show in 2 days.

The venue: A Theatre style room with 2 projector screens.
Vmix's role: To live stream out via a LiveU Solo (bonded 4G) & bring in a Vmix call to one of the projector screens
Video Setup:
- External #1 AJA IO4K port 4 SDI out to LiveU
- External #2 Blackmagic Ultrastudio Monitor to screen via SDI then Decimator HDMI out to projector screen

Audio setup:
- Ext. Mixer Ch1 Lapel 1 via Presonus StudioLive AR8 USB mixer
- Ext. Mixer Ch2 Lapel 2 .....
- Ext. Mixer Ch3/4 Lapel 3 .....

<<These are all Muted on Presonus Mixer >>
- Ext. Mixer Ch7/8 (Super Channel set to USB) Which is the return feed from Vmix via USB. So I can monitor the mix from the headphones of the mixer and not the PC.

The Issue: The presenters in the room get an echo or loopback. I'm sending the output of the Vmix caller to the room via Ext#2 and I dont want to mute the presenters in Vmix otherwise they wont be recorded or streamed.

Do I assign Ext#2 to Bus A and Ext#1 to Master? That way Ext#1 gets everything and Ext#2 gets everything minus Ch1, 2 & 3?

I've tried this but I think I'm doing it wrong. Also, the BM UltraMonitor has a ghosting sound to its Audio. I've troubleshot every possibility to rid the ghosting with no luck. The AR8 mixer has Master & Control Room Out, that im not even using. However the room only has HDMI audio in so analog is out of the question.

Any advice for a setup to try would be very much appreciated.
Attached is shot of the AR8