To Report Back some positive News:

As my flight was ticketed on OS using the OS flight number, despite being operated by FB I did get a voucher for lounge access in Varna.
It took a little bit of work getting the voucher but it wasn't hard once I asked the right person. (I wasn't given it at check-in but instead had to go back to the entrance lobby of the departures area and ask at the small Austrian Airline office who then walked over to the check-in desks to get me one). I'd asked on the way in and they confirmed this would be the case so it was definitely policy.
Due to time constraints I didn't actually have time to use the voucher so I still not sure what the lounge is like.

Secondly, the check-in staff member didn't provide any *A priority tags (as expected). I'd brought along the ones from my inbound flight and attached them myself. They weren't too happy to do this and tried to remove them twice, but after a bit of pleading (twice) they let me keep them on.

My baggage was checked in all the way through from VAR-VIE-LHR whereas strangely, colleagues to different destinations (on-wards from VIE) still on *A had to recheck them in VIE.

VLML/VGML meals didn't exist on-board the FB flight for some reason despite being in my booking. I did get a meal thanks to the very hospitable and friendly crew.
A further bonus, I even got mileage credit for the sector!

With regards to priority check-in, there only appeared to be one check-in queue for the flight - and only one flight leaving the terminal so no other desks open. I didn't want to risk embarrassment by getting it wrong since I was with a few colleagues so I didn't attempt to use this service.