With Monique as a zealot who killed her own aunt and Davina out of it and the remaining key witches being dead, what better replacement then the leader of the coven herself? Who already has a connection to Klaus (the main character) Marcel and Davina.
To be a series regular, one must have some sort of meaninful connection with at least one other series regular. If it weren't for Rebekah's departure, Genevieve would already have four. Kieran's in second place with three (Marcel, Cami and Klaus) Everyone else has one or two.
No show is perfect, so I see it as nothing more than a little shark jump.
The only logical answer I can congure up is that Nadia knew that aggression and violence against Damon wasn't going to help in finding the body. Plus, Stefan was in the room. I'm sure Nadia can take them both but why waste time? Also, Mia was a traveller, and they are always on the move, so Nadia probably had a tight schedual, as did Mia.
I'm sure that if it were just Damon and Nadia in the room and he did something to really set her off, Nadia would win in a heart beat.