I think the answer has to be how he appeared to her when he visited Narnia, and how he behaved. Clearly he was charming, pleasant, and appealing then, entirely suitable for a queen to consider marrying.
I guess they didn't get round to discussing theological differences!

Well, Edmund tells her that they were sort of surprised that she took him for her lover... I don't think he thought he was all that great either when he first came to Narnia, or perhaps it was his protective brother mentality.
Either way, I didn't get the feeling that she was "heartbroken" after discovering his true colours. Of course she would be relieved, but I never got the feeling of her being heartbroken, which makes me think she didn't really fall in love with him. Perhaps it was a crush, or maybe she thought she could come to love him in time.

That not a question I've never really considered, I certendly don't think Susan loved Rabadash I read over that part of the book again and she mentions how when he was a guest in Narnia he behaved " Meekly and Courteously" and performed " Marvelous feats" at the tournament Peter held for him and once in Tashbaan and seeing him for who he really is she says it has her "Folly" for ever showing him so much favor.

So I think that she was definitely infatuated with him at first then realized her mistake after seeing him in Calormen, I used to wonder why it took her three weeks in Tashbaan to see his true colors when Edmund seemed to dislike him from the start now from what I've heard from people I've talked to infatuation can make a parson blind to someone's faults for a time but still given how short if a fuse Rabadash seems to have with the slaves Seems a bit she wouldn't pick up on it sooner but perhaps this is a moot point I just thought it was a little odd.

As for the marriage of convenience idea I don't think that any in Narnia would even entertain the idea of any one marrying for solely a political purpose.

I agree. As Hwin says to Aravis, "No maiden is forced to marry against her will in Narnia."

Agreeing to a marriage that is good for diplomatic relations between two countries isn't marrying against will if they were to be willingly wanting to perform their duty as King/Queen for their country. But I agree with you in that it doesn't seem to be the case here as Susan was clearly infatuated, but it isn't really too unreasonable to think that better relations between Calormen and Narnia mightn't have played in her mind because after all, this was Susan, who was the most logical of the four and always thought things through. I don't think she would act on a whim or a fancy. If she went so far as to consider marriage, she would've thought everything through.
While it mightn't have happened in the Golden Age, I don't think that all the Telmarine Kings married for love. And if the Kings and Queens of Old had to marry as part of their duty, they would have for their country, and not out of force. But it never came to that, and I don't think Aslan would've ever let it come to that. I think in the fifteen years that they ruled, they never fell in love, gotten married or started a family and things because Aslan knew they had to return to their world someday.

As for Rabadash, I think he was like those classic psychopaths who can be excellent manipulators with all the charm in the world, but really vile with no conscience or remorse underneath it all.

I don't think that Rabadash was necessarily a psychopath, merely an over-indulged and horribly spoiled boy who was all too used to people fawning over him and whose father had spent little or no time with him let alone given him much of an example of how to be a monarch. Or maybe I am wrong also. Check out what the Tisroc has to say to Ahoshta about Rabadash who has 19 brothers. There has been an actual Ottoman Empire Sultan who murdered his 19 other brothers on accession to the Ottoman throne. I doubt Rabadash the Ridiculous was something like Selim the Grim. Actually Rabadash's plan to seize Archenland might have been good strategy, if it wasn't for Shasta and Aravis' timely intervention, and the sheer impracticability of getting to Archenland with an openly military force without being detected.

Agreeing to a marriage that is good for diplomatic relations between two countries isn't marrying against will if they were to be willingly wanting to perform their duty as King/Queen for their country. But I agree with you in that it doesn't seem to be the case here as Susan was clearly infatuated, but it isn't really too unreasonable to think that better relations between Calormen and Narnia mightn't have played in her mind because after all, this was Susan, who was the most logical of the four and always thought things through. I don't think she would act on a whim or a fancy. If she went so far as to consider marriage, she would've thought everything through.

The problem with marriage amongst Royals, including our Narnian monarchs, is that there are going to be national strings attached no matter what, and whether the couple concerned like each other or not. A royal marriage is almost like a statement of political intent, and all too often in the past there has been a conflict of interest or other implications when such marriages are formed. To cite a RL historical example, Marie Antoinette, the youngest child of the Austrian Empress Maria Theresa was only a child of fourteen when she went to marry the French dauphin, and it seems she liked him well enough to be his Queen and mother of his children. But as L'Autrichienne, Marie Antoinette became the hated target of the French Revolutionaries, who at her trial, accused her falsely of all manner of atrocities, some of them committed by themselves.

It isn't too far-fetched to say that before World War II, very often Royal personages had limited choice, if any, in whom they were allowed to marry. It has often been this very lack of choice that has determined the progress of history. And the youth of Marie Antoinette when she went to her very expensive and glamorous wedding, also suggests a comparison with all of the main female characters we meet in in HHB (except Hwin and Lucy). That is, how much of what a young girl might want and fear in the early teens about how men look, and how little a young girl thinks about other complications such as "reasons of state".

At the end of LWW we find that Susan and her family had been in Narnia for 15 years which would make her a woman of 26 or 27 before her return to her original age. At the age of 12 at the beginning of the books, Susan was very practical and liked reading. But later on in VDT we find she is "no good at schoolwork", and by LB she is "too keen on being grown-up". So it is easy to suspect she might have been reading too much romantic fiction at the age of 12 and not enough schoolwork. Also to deduce that Susan's main aim in life was to find a suitable marriage partner and establish her own family. At the end of LWW we read that she has been sending out ambassadors and pictures of herself to other Narnian countries, including Calormen as well as receiving ambassadors and pictures in exchange.

And the undeniably handsome Rabadash takes her up on it, arriving in Cair Paravel to see her. Why? To see if the picture matched up to the reality, like Henry VIII and Anne of Cleves? Edmund, who knows a thing or two about meeting good-looking strangers, suspects him all along as a fake. But Susan, who knows even less about Calormen and its ways than does the Tisroc about post White Witch Narnia, isn't really looking at that sort of thing when Rabadash comes to visit at Cair Paravel.

And Rabadash does have the nous to play his part so well whilst in Narnia that Susan was impressed and agreed to visit him in return. In my opinion Susan was merely "testing the waters", as young girls sometimes do, dreaming of the proverbial rich "tall dark and handsome stranger" who in pre-WW2 times would carry her off to married bliss. I don't think she initially suspected that Rabadash took her reciprocal visit to Tashbaan as an acceptance of marriage. In Susan's upbringing she'd need considerably more than a week's visit and the exchange of pictures and ambassadors to decide if someone is a suitable marriage prospect, regardless of how attractive they might first appear.

Which do you think is more important to HHB: How Susan feels about Rabadash? Or the real reason why Rabadash might want to go to Narnia to see her in the first place?