We could only call Merrill 'playersexual' if we had proof of it. Like if she told FemHawke "Oh yeah, I'm totally only into women", but told ManHawke "Oh yeah, totally only into guys". As you said yourself, we have nothing to define her romantic inclinations by, except her entanglement with Hawke.
In other words, Merrill's sexuality is purposefully left vague. That is not playersexual.
At the root of the concern is the insistence of some players that their perception dictates the fact. For instance, if someone doesn't come out and state outright that they're attracted to members of the same gender, then they're heterosexual by default...and them stating differently later on must mean they've been ret-conned for the player's benefit.
As you state, the sexuality of some characters was left vague so it was open to interpretation by the player—but some people don't want interpretations, they want labels. Bisexuality is inherently indecisive and character-breaking in that case, since the perception would then be that the characters are altering their sexuality...like a light switch that goes from "completely straight" to "completely gay", since those are the only points that exist.
I get it, even if I don't agree with the assessment.
Insofar as what we're doing for DAI romances, we've yet to discuss it. This particular discussion usually goes nowhere good, however, since there's a lot of ignorance and indignation that arises whenever the issue arises on these forums. I suggest that, if anyone has anything in particular to add, they do so thoughtfully and with some consideration in mind.