I don't see a situation that makes sense for either a good, neutral or evil chracter in telling Minthara something like you suggest.
People like Minthara would take any information you give her and then decide how she could use it to her benefit. "I like you" would make her think how it can be exploited.
She would consider if she can use your emotions to solidify her position and power.
Why not? Let him use it, I don't mind
Hmm, but what is the reason for PC to even work for the absolute when it is pretty clear they want make us their puppet and maybe even turn us in mindflayer during invasion day. Minthara is a total stranger. Unless it is a blind love at first sight, PC would rather try avoid working for her or absolute at all cost.
On the one hand, you are right, but now there really is no reason to go through the evil root except "I liked Minthara", there is no ... yes, nothing at all except her in this root. Helping the absolute looks so illogical that to tell Minthara directly why the main character is doing this looks adequate. Then it will make at least some sense.
By the way, Minthara most likely does not know about the tadpole in her head, and an honest conversation can make her think about the fact that she is also being deceived and manipulated in the second act. After that we will be the only ones who can somehow help protect her from both the Absolute and the clerics of Lolth .. in general, there are many things that Larian can think about
