I recently sat Shiva. Not sure why anyone who knew neither the nifteras not the avelim would have come to be menchem avel (don’t remember any such people) but anyone and everyone who came was and is appreciated.
What is and was awkward was people who I knew vaguely coming and just sitting and sitting there after the conversation ran dry (usually because they were waiting for someone)
IMHO if you go to be Menachem avel with your wife, make up with her to wait for her on the porch/front of the house if you finish first. And if you primarily know one of the aveilim but due to the seating situation end up near one of the avelim you only know vaguely and the conversation has run dry, just get up and say “let me see if can go over to…”