I don't think Ward is involved with this. He's off working on the movie or something as I recall. He's stepped down as head writer and according to Muto apparently 'contributes to the series' by helping out with outlines or something.
It'll be interesting to see what they do, but I'm not so sure this will actually count towards canon in the end. The show is doing more arc style storytelling recently, and we'll see if Ward stepping off from writing will hurt the show or not.
Abadeer is a demon, so Marceline is at least half demon. How and when she became a vampire on top of that has not been explained, but it's pretty clear she wasn't human when she was running around with Simon as a child. She had fangs, blue gray skin, and no marks on her neck. She might have been a vampire then and the two puncture wounds in her neck might just be something like stickers, or a tattoo, or something she does because she likes the look.
It could just be that she ages, but reeeaaallly slowly, or it could be that those born vampires will age to a certain point and then stop, or it could just be that she was turned around the time she became a teen.
I'll give this a shot. Maybe they'll manage to make it make sense and mesh well with what we've seen of her past before, and maybe they won't. Pretty sure Ward is busy with other things and won't be extremely involved with this. He probably has veto power and might consult a little, but the film is probably his focus and priority while the B team takes care of this.