She had a 3-year relationship and....
Allow me to correct: She had a more than 11 years relationship with the prince. She met him at a study visit to England when she was 19 years old. Quite early, she became pregnant by him because her daughter Aya is already 11 years old. That means for me at first, probably it must be that to "give a proper thought" comes with pregnancy, but not before. Means: possibly she was not so well informed about all potential consequences which followed, as you assume. From my point of view it is to assume, that the prince because of his young age itself was not aware of the consequences to him at the beginning of the relationship. Both acted only from loving feelings for each other and had forgotten the real world.
she also converted to Islam (which she would not have done without research or if she didn´t apreacciate the teachings of the Prophet).
What makes you sure to assume this? In interviews, she said she converted to Islam for him. ???? For me it seems more an acting out of blinded by love. It seems possible for some people to convert without knowledge (it must not have been the case with her, is only a mental stimulation).
The prince told her that they are going to marry and obviously they did marry under muslim law ...
One article claimed that they tie the knot in Egypt after the birth of Aya. In another article stood, they held their marriage in Lebanon before Aya was born. Well, for sure Mrs. Candice converted to Islam before marriage. She would better to ensure here that the marriage contract contains an exclusion clause against polygamy. She has apparently forgotten (or simply had not been informed that she can claim this if she wants). It is not clear to me whether this marriage was registered later in KSA or not. If not, then marriage has been hidden by the prince obviously.
but did not inform her about the expectation of his familiy to marry a cousin. (Which he has probably known for a long time, some Arabs tell me that their mothers started looking for a bride very early - so this was not a surprise to the Prince.) As I said,HE knew his culture much better than Cohen Agnine, it was his responsibility to teach her.
Maybe you have right here. He neglected the special requirements in a bicultural marriage. However, he did have the option of polygamy and regarded the demand of the family for an "official Saudi wife" as normality, about which there is no need for discussion. My opinion: It is more advisable for a woman to get necessary informations from herself - regardless of the husband - and not to wait he offers. In addition, there is the danger that he tells his wife things that only correspond to "his truth". Also, I wonder just why she had marry a prince of the Saudi royal family, if she strictly rejects polygamy?
Also he was not a Crown Prince or important member of the RF (there are countless Saudi princes and princesses that even locals don´t know all of them.) and therefor it was not obvious for an outsider to know that he has to marry a woman chosen by the family. Usually the Crown Prince and important royals have to select a special bride with good family relation,noble status,good looks,healthy,pristine reputation etc. but the less important royals can chose a person they love.
That would contradict your statement above in part. You wrote that Arab mothers early look out for particular and acceptable brides for her sons. This means it is a social standard in society that marriages are arranged. Even with non-royal and non-ruling Arab families. In the Gulf region its true and actually rooted in the tradition. In addition, preference is given to marriages within your own kin, your own tribe, your own nationality, your own religion. On the Arabian Peninsula pure love marriages do occur, but represent not the majority of all marriages.
When you come to my country it is my responsibility to tell you what is right and what is considered rude by our society,where you need to be careful and which places are dangerous to do.If I don´t do that I would be partly responsible if you get into trouble because I can´t expect you to know the subtetlies of our culture and way of living.
Even if I trust you fully, I would teach myself first what is important to know and proper in your country and society. I would not wait, that you explain to me, how I have to live in a appropriate manner in your culture. I take care of myself first!
I joking! I´m harmless! 
Your idea would not works in this way. Also we are talking about a Saudi prince and KSA and not about UAE and Dubai. Right?
