Ad (1) I may be missing something (and in this case honestly missing it) but where do you get that the people love us? We have the weekly report with their "attitude" given as a number, but as far as we know that is to stop them from rebelling which is a long way removed from actually "loving" you. And we probably do not even have this number maxed out since that would keep us from meeting some girls. People will go along with you but will they care if you were to be replaced? But yes, it is true, as long as you provide the necessities, average Joe may not be interested at all in political shenanigans. However, the (perceived) legitimacy of a ruler is something people in different ages have often cared about.
Ad (2) Conflict with North may be bad for the queen but it would also be bad for MC. History is of course full of cases where the fact that it would be bad for both parties did stop escalation - and it is also full of cases where this did not stop people. Being too sure that somebody will not hurt you because they would also hurt yourself may easily backfire. I don't care about what happens to the queen in this scenario, but about what happens to MC. And the truth of the matter is that Taina is a way to strengthen his position. It may not be needed indeed, but then again it may.
But your next point is indeed the crux of the matter.
Indeed it could have (even though I do not subscribe to "just killing her" as the most satisfying conclusion), just as the plot itself could go to hell and back as far as I care. But as you said, these are dev decisions. And a dev does not need an excuse to do as they please with their story. There are no excuses needed. This discussion in my remembering started as "the story as presented by the dev is illogical within the world" and I will hold that it is not.
I very much doubt they care for Taina at all. That's just the impression they will give. She is just another pawn in the game, in this case one that is protecting you. And Dex would love for the pawn to be gone but cannot say so. And yes (referring to a different post), these kinds of things have happened even within the wake of an invading force or other hardship. Dex is scheming and interested in his personal power. I doubt he even gives a shit whether the horde can be repelled as long as he sees some future where he thinks he has personal gains from it. He most probably wouldn't, but these people tend to fall into that trap easily.