This is a worse idea than making her our consort. If we're going to make someone a fully fledged night troll then we will want someone who's actually going to *want* to propagate the species, someone who is going to actively attempt to spread our kind through the world again, without our constant babysitting.
One word:
instinctsAlso, You overestimate effects of sexual orientation, many medieval nobles were homosexuals and had kids with their wifes.
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Reasons to talk Alice into becoming a night troll
Roleplayish
1) We consider her our friend, no other gift is truly suitable for a friend, you don't make friends dependable on you and your family. Leaving her as a weak mortal servant is not a friendly act at all.
2) We know that Foghorn expects us to turn his daughter and we view him as an ally.
3) We trust her to not use her newly gained power against us or our family
Storywise:
1) It's a matter of taste, but in case of our death I prefer to play lesbian\bisexual ranger to a priest with tail
2) I'd love to see love\hate relations between her and Gwen
Powergaming:
1) Alice knows that area quite well and will be much better hunter due to her ranger skills
2) She has a nice family, father and sisters will help her. As well as the Tigerwoman
3) She is skilled in magic and can improve in that field quite nicely, thanking to nice theoretical knowledge from her father
4) She will get along with our female consorts much better than any other night troll
5) We need reserve option in case if we die. Unborn child isn't good enough
Bonus:
Alice may consider that a nice offer because
1) She knows that her father expect that and it's better to not disappoint him
2) She'll get immortality
3) She will have only daughters
4) She may treat her "husbands" in
any way she like. Up to: convert, fuck, kill
5) She will become a matriarch of a strong Night Troll branch
6) She will be less pretty (read less desirable for males)
7) She always looked at us with some awe, looks like she considers being a night troll cool (being part of harem is a totally different thing to being a real night troll)
It's unlikely that she'll ever get a gift better then that