Not really if she is bisexual, she is bisexual. That logic is the same as saying she is straight but likes women at times. Or she is a lesbian but likes men at times. That is why it erasure to put everyone ambiguous and also everyone bisexual.
Uh... okay, but I personally know people who ID as bisexual AND demisexual, so nooo it's not at all like a women who is straight 'liking' women sometimes. Like, at all. :/
Also there are people (like me) who might technically fall under the description of multiple things-- There are folks who would call me demisexual, and bi or pansexual too, but I don't call myself any of those things because I don't personally like the labels. I call myself queer. That doesn't mean that I can't look at a character like Dorian or Merrill and feel like they are providing me representation. I don't expect Bioware to make a character who is functionally demisexual/bisexual but calls themselves 'queer' just to feel like I'm being represented.
So I stand by my feeling that Merrill, though technically "bisexual" by WOG, has nothing in her characterization that would contradict someone experiencing her as also demisexual, if they chose.
EDIT: Maybe it's because I'm over 30 and didn't grow up with ANY representation of any LGBT characters like... at all, that I'm just as happy with a gay character as I would be with a bi demisexual character. I'm just so goddamn happy that Bioware gives a crap about the LGBT community at all.