Yeah, having more options is the point, even if some of them don't work. Just like trying to remove the tadpole.
Romanced ascended Astarion can turn you into a spawn even after you choose Bhaal, so Larian already decided that Bhaal can't do anything about it or doesn't care. And it doesn't matter in the end, though that might just be because the endings are so rushed. Otherwise Astarion would at least try to do something before you go on a killing spree, even if unsuccessfully.
And of course it's an unhealthy relationship. No way he turns anyone into a full vampire. But in a game where you can turn into a squid and kill yourself, or lose your mind to murderous urges, it would be just another tragic ending.
Bhaal would probably see it as a Win, not only would his chosen spawn now be immortal and more powerful btw, but he now has one that has knowledge of the Vampire Ascendant ritual and the secret of becoming a True Vampire, all Bhaal who have to do is get the Durge to become a full Vampire by taking control, then the Durge can then replicate the Vampire Ascendant ritual on a bunch of fellow Bhaalists, I mean it's a ritual that kills people, something Bhaal himself would approve of, plus Bhaal isn't an idiot, guy came up with the Bhaalspawn idea in order to resurrect himself, in the case of his death
