Marcel obviously has absolutely no knowledge we've learned and gleaned from The Vampire Diaries. That is part of what watching The Originals IS so fun for as we, as fans, get to live through Klaus, Elijah, and Rebekah (even though we used to hate them--at first--except for Elijah, who seems to be perhaps the most "stand-up" and particularly "normal" vampire to have ever existed; keeping Elijah out of episodes hurtsthe shows ratings badly, I have no doubt) to some degree as we know perfectly well how absolutely dangerous and impervious they are.
If Marcel knew, I doubt he if ANY vampire would truly consider giving a witch (especially really powerful teenage ones) that knowledge. If they learn what tree it is, perhaps they can search for them high and low using magic via a herbalist/witch, but you never know they might be able to get nature to coax one back into existence as long as they are willing to pay the price--BUT, if it WILL get rid of ALL vampires then I'm more than sure they would pay it!
It would be obviously stupid for Marcel to even begin doing anything down that path...BUT, I think Elijah may have given Davina a clue as to what is going on. She should realize that she removed and replaced the dagger, YET he still came back. So obviously the dagger is NOT what is causing the primary cause of "comatose death-trance". I would think she'll assume that either a spell was put on it right before hand (though she may be strong enough to feel or not feel a trace of lingering magic up to a few days or weeks--who knows... Otherwise, all that is really left is that something--like vervain--was placed on the dagger before hand.
Don't be surprised if Marcel (or one of his guys) goes to The Originals new house for a visit, but really there "looking" for anything they may be using on those daggers. Beyond that he may press them for spell creatiion knowledge or details about their past through Rebekah I imagine (with a semi-fake relationship that could get him killed...).