I think it's a bit of a stretch to see killing Adramahlihk as being transphobic in any way at all. It doesn't really matter what gender the demon does or doesn't have, the body isn't Adramahlihk any more than I am the gender of the Mean Black Car when I'm driving it (for the record, Mean Black Car is a Ford Mondeo. I don't know its pronouns because I don't speak grr). The (presumably formerly good) doctor is simply a vessel, nothing more.
I get the fact that Daeva was used by Adramahlihk a suit in order to work with some humans, just like Janus was for demon of lies, but why the model for demon is feminized? It's one confusing...
I'm going to kill Adramahlihk because he's trying to take control of ma girl Lohse, rather than being transphobic, or whatever. Also the idea of genderless demons being still able to reproduce with whatever creature they deem worthy seems to be a complete stranger to you, I see. The demons could be generally genderless most the time, when not reproducing (unless they really love to reproduce with someone every minute, well those are probably weirdos even in a demon realm), but when deciding to spawn some offspring, they could temporarily change into the opposite for the person of their interest. Like yes, there is no evidence of such thing in the world of Divinity, but that doesn't mean, that it couldn't be possible. There's also no evidence against it. Just because Malady is a half demon, doesn't mean, her daddy, or mommy couldn't be a genderless demon, temporarily changed to whatever fits the other half of her parent. Because honestly we don't really know a thing about her, she has more secrets, than the US government. We're talking about demons in a world, where Polymorph exists, you know. If a low level Sourcerer can shapeshift parts of their body, why not a powerful demon/archdemon such as Adramahlihk?
My whole transphobic argument was entirely a joke. Regarding the demons : all named demons in other games (Succubus, Demon of Lies, Corvus, Samael, Asmodeus, Izikiel(or something) had specific gender and not "fluid" form, so I'm not sure about genderless.They were created by Lord of Chaos (their God patron) and considered as a race so there must be some form of similarity with other races from Rivellon (or Namessis).
Demons in the Divinity universe exists outside of the boundaries of planes that most if not all of the other species are bound to, so it's not a stretch to consider they may not at all be bound to a single gender, or any gender. It's all just vessels made of bone and flesh.
In Divinity universe demons don't need vessels and requires only to be summon. There was only two times when demons used meat suits : in case with already mentioned Adramahlihk and with Demon of Lies. Last one was created from part of the soul of Lord of Chaos in a form of the sword with purpose to corrupt it's bearer, break his will and tranform him/her in Demon of Lies, who, in turn, will summon the real Lord of Chaos.