I reckon the queen HAS to be somebody of royal blood. So if the prince was the only surviving royal and he married any woman, whether she was a commoner or knight or noble such as Luserina, then his wife would not become queen, but if they had a daughter, then she would become queen. If he only had male heirs, then we would have to wait until one of them was old enough to marry and bear children, and Falena would have to remain without a queen all that time. They might give the prince a title such as lord regent or something, or he might become the Commander of the Queen's Knights by default if he was the only royal left, but I don't think they would allow the title of king.
So if Ferid were to outlive Arshtat, then he would not become king, he would continue as Commander of the Queen's Knights until Lym got married, and then he would have to step down and let Lym's husband take over as Commander.
If Sialeeds were to outlive Arshtat, then she probably could take over as queen, but only if Arshtat had no children. Same with Haswar, I suppose.
But if a male heir were to marry a royal princess from a foreign country, such as Armes or Obel, then that could be interesting. She would end up bearing a daughter that could have a claim to Falena.