Unless it expands like the chimneys in The Santa Clause. 
That... doesn't make much sense to me. The Eluvian appears in the Crossroads, the place where all Eluvians appear, and this is a bad thing? How? I mean.... Morrigan's got the same thing going on. Morrigan's knowledge of the Eluvians is better, mainly because she had an entire book on them for a time, but I wouldn't say that she has some idea of what she's doing with them either because she doesn't even really understand the Crossroads entirely.
Many people dislike Merrill because she is a Blood Mage, even though, time and again, the series has shown that Blood Magic is just magic and as such is not inherently evil. In fact it is useful. Templars use it to track down Harrowed mages, Grey Wardens use it in their rituals, etc. We have had an ancient Elven God of Rebellion, a runaway Tevinter mage and a Witch of the Wilds tell us this.
The only drawback to Blood Magic is that it reduces one's ability to connect to the Fade. As for its mind controlling properties, we have flasks and grenades that can induce confusion and mass panic and those are all okay for some reason. It is okay to have a chemical weapon that results in some form of mental insanity but magic that can control minds ? Talk about hypocrisy.
From an intellectual standpoint, all Merrill wanted to do is to understand the ancient Elven culture by reconstructing an ancient Elven artifact from nothing. This is not very different from an archaeologist investigating a dangerous ruin in an attempt to recover artifacts to understand the culture of the people that once lived there. Both have their own dangers.
As for possession, there was no point, except the one in the Fade, where Merill succumbs to promises of a demon, turned against people and started attacking them. Then again, almost everyone except Hawke did that at that point. She had plenty of opportunities to succumb to demon possession in Kirkwall but she didn't. She mostly minded her business and helped Hawke. She did not even attack her clan, she just decided to move out.
When push comes to shove, Marethari was the fool who let the demon possess her. She is an Elven Keeper, someone who supposedly has control over her magic. Don't tell me she cannot hold off and attack a pride demon, at least until Hawke & co came ? She could have even brought some Dalish Archers as backup and banished that demon, Audacity.
I hope we see her again in the future. Adorkable blood mages are hard to come around.