Ok after futher testing I found out that you get the quest box choice + the strength one by threatening to cut her head and then lose your intimidation roll.
Did it like that and game gets bugged out.
Ofc, both hairs are available, but Mayrina vanishes even then when talk about deal with hag is over. Also Hag Ethel vanishes along with Mayrina.
On top of that the Questlog and the Save Mayrina Ques is also completly deleted from log. not hidden, plainly deleted.
Either its not a finished Questline having this solution and Larian didnt wrote another ending and made cutscene and anything onward from that leading to Baldurs Gate and Ethels Sister Hag M.
it is really a bugged out Quest then.
you forgot to mention that after losing intimidation check you need to choose Power over Mayrina to get the selection screen. Then saying goodbye to Ethel in aany of the 3 dialog choices. This leading at least some of your companions to DISSAPROVE your Action.
So why lose the Intimidation check anyway. Just let Ethel go with Mayrina by only chosing POWER option.
You will get both Hairs in any way bec. it seems its a bug in patch 6 that you allway get both hairs by doing so.