Yes, it's an interesting discussion! Like the author of the topic, I have a number of questions. I hope it won't be off-topic, since the topic isn't defined by specific boundaries. So:
Let's remember the end of the original game. Isabel is freed. Zehir says that Isabel will be sent to the Silver Cities and isolated, etc., but in the final video, we see that she is with Godric and, by all accounts, is already Biara. So, the question is: how did they replace the rescued Isabel with Biara? When? In Hell itself? But where did Railag get the real Isabel, if she wasn't in Hell? Could it be that three out of the Magnificent Four are clinically insane, and Isabel didn't notice, but Railag did, took her somewhere, and didn't care about the future of the world, knowing that Biara would be on the throne of the Empire, even if in Isabel's guise? It doesn't quite add up... :confused: :eek:

Based on the videos from "Lords of the Realm," I can say the following:
1. During Isabel's stay in Shi'o, in addition to all the sacrilegious acts committed against her by Kha-Beleth, she was also subjected to the deprivation of a part of her spirit. As I understand it, this spiritual component taken from Isabel is what allows Biara to impersonate her without killing her first (because, apparently, a succubus can only transform into those she has killed; for example, into Beatrice, and, apparently, she also killed Princess Shadia at some point).
2. The 4 heroes save Isabel's physical body with 1/2 of her spirit, but leave the remaining 1/2 of Isabel's spirit in Shi'o.
3. Then there is some time period not described in the game, during which, let's assume, Isabel recovers in the Silver Cities (let's assume, to justify Zehir's words and to try to resolve the inconsistency for you). But no matter what, the mages cannot make a whole spirit out of 1/2, and, excuse me, they simply cannot perform an abortion on the queen, because the coming of the Dark Messiah is inevitable; one of them (the mages) - Sar-Shazarr - prophesied it, and therefore the mages themselves are not very confident that they will succeed.
Seeing that everything is useless, in my opinion, Railag could not do anything other than steal (or, with the consent of the mages, take with him) Isabel, who really doesn't want to become queen again (she expresses this unwillingness to ascend the throne in "Lords of the North"). Because what's the point of leaving Isabel with the mages when it's clear that they are incompetent in this matter?...
There could be objections from Godric, saying, "I promised Nicholas, and so on, Isabel must ascend the throne, and I don't know what," but he probably quickly shut up, seeing Isabel's unwillingness (not only to ascend the throne but also to return to the country that she plunged into the abyss of external and internal strife and where half the population hates her), reinforced by Railag's persistence.
The lack of significant, principled objections from Godric and Zehir on this matter can be explained by the following - does anyone want the Dark Messiah to be born in their country? Who knows what will happen on the 1st day of his birth? Maybe an explosion like Ice Blade + Armageddon's Blade, or something else equally terrible?
As for Faydaen, he apparently decided not to get involved after his option - to kill Isabel - was unanimously rejected. Moreover, unrest began in his country (as evidenced by the single-player scenario), and now, whether he wants to or not, he must distract himself from saving the world and save his homeland.
4. On the other hand, we can assume that Isabel did not undergo treatment in the Silver League, that Zehir's proposal was also rejected for one reason or another, and Railag took her with him immediately. Then we skip that part of point 3, which talks about treatment.
5. And now, let's imagine the following picture. Railag and Isabel leave for the Dark Elf lands; Zehir remains in his country to fulfill his duties as 1st in the Circle; Faydaen returns to his home, giving up or not giving up, but considering it pointless to stay; Godric rushes to the Griffin Empire to place Andreas on the throne or do something else. And then, let's say, some time after their separation, Godric is on his way to Talon (Talongard) or somewhere else (it doesn't matter where), and he meets... Isabel. That is, Biara in the guise of Isabel.
The bitch starts pressuring the knight to be true to his word, saying, "I've changed my mind, I've gathered my strength, forget about Andreas, he's still just a kid, I'll rule the country myself, and you help me, you promised my husband, remember, etc." He agrees because loyalty to his word is more important to a knight than anything, even the threat of the Dark Messiah.
And Railag and Isabel are already on their way to the Shadow League or have been there for a long time. And they have no idea about this.
Biara ascends the throne and first surrounds herself with fanatics, for whom the Higher Goal of the faith in Erathia is not important, but the process of fighting for it in all its bloodthirsty and ruthless diversity. Laslo becomes a general, Alaric becomes an archbishop (thankfully, Rendell stepped down from his post during the reign of the real Isabel), Flammschrein (Holy Flame, Alaric's city) becomes a sacred city, in which Biara places the crystal with 1/2 of Isabel's spirit (so that no one can take it away).
And a terrible tyranny and lawlessness begins, leading to a world war, in which absolutely everyone gradually becomes involved.
And by the time Railag and Isabel find out about the deception, they can't just show up in the Empire and say, "She's not real, and here I am, the most august queen of the Griffin Empire." Try to prove it... and how? With the Heart of the Griffin, which could be used to deport Biara to Shi'o, they no longer have (this is evidenced by the video from "Lords of the Realm," which says that the Heart of the Griffin is with Biara); apparently, the cunning succubus or the Lord himself stole the artifact from the 4 heroes during their stay in Shi'o.
Therefore, they have to act consistently.
P. S. Finally, I would like to say - Comrades, let's be less critical of the scriptwriters! They are not fools and clearly tried for us and thought through every detail... Let's better bash the artists and designers, as well as everyone who made the images of many units so idiotic... I'm done, I'm done, I understand that this is already another topic... :)