Or each of the other True Souls have their own agendas as is the case in most evil cults. Which is why their temples are fewer then those of Good dieties, and why their leaders are so touchy, paranoid, etc. Who knows when an underling will stab you in the back, take your place, etc.
Minthara since she's the focus for the op. is a drow who enjoys toying with people while fulfilling their base needs, and desire. The sleeping with you is fulfilling her lust, and toying perhaps with your emotions, making you think she trusts you, loves you. Giving a gift continues along that line, with her playing with you, and testing your reaction. That way you'll be all the more shocked that she betrayed you and is willing to sacrifice you, which is all the more sweet in her eyes. All typical drow relationships, which is why there is no real trust in drow society. In fact it's mentioned in several books that their is no word for trust, or friends in the drow vocabulary.
Yes she would be unless the Goddess wants her wishes kept a secret, wants you to prove to Absolute that you are worthy or not. Pitting you against the others for her sport, amusement, etc much like Lloth does, as do other evil gods. So Minthara may not know as much as Guts, or may know as much but know different things. So both think you are not spose to become one of them, while the Absolute is testing all of you. Making you prove your worthy.
Now perhaps she did have feelings for your character, yet to show her devotion to her goddess (usually Lloth) the Absolute, and gain even more favor which means more power for her, maybe becoming a high priestess instead of a despicable goblin, she is more then willing to do so. After all she is drow, they've been doing it for who knows how long for Lloth, it is no different when Lloth is replaced by the Absolute.
How/why/when/what Absolute religion. As stated by SH a new god would have an uphill battle. A god who may have been a demigod as of yet unknown, or one that lost all their followers, or is from the pantheon [spoiler] the book found in the first ruins of Jergal [spoiler] a god especially an evil one would at first only grow slowly. Goblins being one of the easiest since they are dumb, voilent, and tend to be avoided by others unless being hunted or subjegated. Plus they are swift breeders, and alot of the time out of everyones way. So goblins give you secrecy, alot of followers, and followers who are easily swayed. From there it could of spread to through manipulation, coercion to other monster tribes. Until the Absolute felt it was time to show her/his hand in a remote part of the world.