I guess narrator-san told us the story from the perspective of Aria. But I doubt it's as clean as it has been described.
@alidan: That's nearly the utter most favorable light to present the situation. Well, I'm sure, this is how the prince sees himself, but why isn't he interested in the fate of the second summon person, Sei? ... I have a more sinister impression on him. A counterpart of your picture, if you want.
He assumed right from the beginning, that Aria is the saintess and discarded MC. Why? Because he liked her looks. It is rather obvious, that he is smitten by Aria, as so far he showed the typical scumbag prince behavior in the shown situation. And he is keeping her away from everything, that might end her dependence from him.
Here are my two cents on this matter:
Maybe he was fascinated by the idea of summoning the saintess for a long time. As you pointed out he might not be in love with his fiance. Maybe because she is too bright for him. It might have been something she did or said when they have been younger or how she has been presented to him. Maybe some fool always took her as a prime example for good behavior. "You should do this. Your fiance can do this easily. You don't want to make a fool out of yourself, do you?" So he doesn't like her and doesn't want to merry her. But how can he escape? The answer: The saintess. If he would merry the saintess nobody would complain. I believe the prince fell in love with the saintess even before they summoned Aria. And then Aria appeared, a much shyer girl in a weak position. Jackpot! And she even cried...while I do believe he did feel remorse at that moment, I do believe that bastard was also glad deep down, because he could console her.
Even though he would never admit it, not even to himself, he wants to dominate her live. From his position he picked up a poor baby bird with broken wing and is doing everything he can do to pamper her ba....to make her life with him worthwhile. What a great guy. Those psychological behaviors and the damage, that follows suite, are not well known in his world, I assume. As such I fear, that not even the prince himself is aware, that what he does (cutting her off, so that she stays the way she is) is very creepy.
Here comes the million dollar question: Assume, there would be a magic spell to transfer Aria's miasma cleaning powers to a knight and send her home. No loopholes attached. And only he alone learned just about it. Do you believe he would prepare everything and tell others right away? No real chances anybody else might stumble about it! It's his exclusive knowledge, that is exists and in which books to look for it or which mage to ask. ... She even cried in front of him, but do you really believe he would do it without hesitation?
I know, I would hesitate. I believe this to be normal. I also hope I would hesitate only for a second. But that second is the moment of truth. The length of the hesitation determines how much of a scumbag that prince is.