What you're describing doesn't sound like very good writing to me, Killing Dallis in Act 1 only reveals that you can't kill Dallis, and you can't kill Dallis because she is important for the plot later on, defeating her is also not necessary for the plot to continue, whereas the plot in BG3 advances with the great expectation that Minthara will be dead at it's end, either by people siding with the grove, or because you've rejected her sexual advances, or because you can't talk your way out of backstabbing her (DC questions aside) If they bring her back from the dead here, it'd be through another loophole deus ex machina that they need in order for a plot important character to be around for some reason, which doesn't seem to be the case nor does it sound like making choices that matter.
I also am seriously skeptical that the knockout functionality will allow for the kind of bifurcation you're hoping for, but as a way of putting a hostile Minthara at Moonrise Towers that you could potentially suborn into your quest, I'm open to, I just don't think that without the "heart to heart" that you have with her during the camp celebration, there is going to be that option.
Defeating Dallis in Act 1 is not possible without some big metagaming knowledge that you can only possess by having decent amount of playtime. It's not a spoiler because new players will never succeed at it; even if you manage to do it; we still are given a little hint towards who she really is, not anything that could be said as a huge information or illogical part of the story.
If you have played Witcher 3, Hearts of Stone, there is a good example of a flavor for second playthrough - we aren't told who Gaunter O'diim is until the very end. During the story, we can see him appear as a various characters in the background (soldier/guard etc.) and that is once again something that a new player wouldn't even notice. Because like I said, it's in the background, it's not easily visible and new player could just assume that it's just someone simliar to him. The truth is, that he is almost all-powerful being who basically wants to keep a track on our progress without being spotted by random people (and us).
Where is it expected that Minthara will die in BG3? That she is defeated though, that is indeed what you could expect when siding with Druids. It doesn't indicate her having to die. Wyll and Halsin do tell you that you should probably kill Goblin leaders; but thing is it might happen. Do you remember that Wyll interaction, where he wants to kill the Goblin captain at the Mill and that doesn't have to actually take place.
Her advances at first aren't because of the fact that she genuinely likes you.
Not being able to talk away your way out isn't a problem at this moment and will be non-existant in the future. Unless you mean the bug mentioned above. Or the intimidation check, which isn't so easy, but at same time it's understandable because she feels betrayed; if you succeeds she calms down and talks to you in very polite and caring manner. Like I said, with option for much less random DC, which will be probably favoured by players, this won't be an issue either.
There is very little reason for her to stay at side of the Absolute once she loses that battle and if we are given a possiblity of not killing her. In 99% of cases evil aligned organisations do not tolerate failure in Forgotten Realms and her being there, if she survives and we fight her, but do not kill; is not very logical.
What you forget is that she seems to have a tadpole inside her head, that is very simliar to our own. The reason for her to appear later is as good as for the rest of Origin characters. On top of this she isn't aware of this thing being inside her, so you have a good reason for companion questline simliar to others.
And if really devs expected what you have written to be the case, why would they even bother to add that cutscene in the end of the act where we are told that she is "our lover and ally" ? If she is just going to die in 99,99999999999999999% playthroughs which is not the case, why do all of this (except just for having another choice). We are also promised to meet her once again.