They do have plot immunity - which is just as well in a game. Also, lore wise, you might have troubles staying in the good graces of the Nord without a proper claim to the throne (bloodline and such) and lose the support of the army. It wouldn't be the first military coup in history.
I doubt the normal people are so loyal to the bloodline, they have enough on their hands to stay alive, so if the MC does a good job, why would they go against him and he got the titel from the queen even without marriage.
The military as well as the normal citizens/slaves have suffered heavy losses do to Megan and Tainas betrayal, why should they go against the MC if he punishes the traitors, which caused the dead of their comrades.
Of course he should also make good use of propaganda to convince them even more.
As for the titel, the queen didn't give a fuck about heredity claims, or the marriage to Taina.
She made it pretty clear what she cares for is, that the districts are in order and that he does a good job.
The west lord wants u dead either way, which is based on reciprocity.
The south lord is a child with no power.
The east lord is ur buddy, which would support u against the queen.
Is he in control of the military though? I mean, yes, the highest ranking member surely is on his side, no doubt about it. But if the military gets unhappy (having to go on a raid almost every day including sundays, selling weapons to mutants and enslaving citizens may do wonders to the morale in a negative sense) that may not be worth much. Especially if the validity of the title gets challenged. Unless MC moves to install himself as a symbol of the Nord, this may indeed be important. Will the military shoot on their own people to defend a president who has just thrown the person into jail who convinced said people that he is legitimate?
Well yes selling weapons to mutants and even raiding on sundays may really not good for moral.
But raiding human settlements in the wastelands and enslaving them, seems to be the accepted norm, for the city at least.
Also most of the old military, which was loyal to Tainas family, is dead or deserted. The new military is nearly entirely build up by the MC and his friends, so i really don't think that they would be all that loyal to Taina in the first place.
Would the people even more so the military(which had many looses) try to rebel for Taina thought, if they would know what she and Megan did and would they do it even if the mutant horde is already in the city and all of their lifes are in danger ?
I didnt say "they are untouchable". My only problem with that rule is it being an "unspoken" rule. It should be a scrict law around that world. Instead of killing they might be send to breeding facilities or given to some breeders but killing them should be strictly forbidden.
If you are at the point where the women or their womb is so special and needed for survival, that u can't execute a serious threat like Megan, which wanted to build something like a tnt filled trojan horse to blow u up, no matter the casualties, than it seems extremly unlikely that women would have any rights at all, they would be used as breeding factories, even without punishment.
And so far there seems no problem with a lack of population, more a problem of resources to keep all the people safe, healty and feed.