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.
