In the middle of the war with darkspawns when the country needs to be united?
I do not think that nobility has months time to haggle over who will be the new king.
BTW: Anora is queen and she is not noble birth.
So "at least" human female mage should be able marry Alistair.
except that in thedas mages do not hold political power, at least not officially. Fereldan is a fedual society, it is not an absolute monarchy where the king is the state. The landsmeet elected their king, similar to the Anglo-Saxon witenagemot. There are rules to politics and politics can be unfair. The fact that the Landsmeet is able to elect their ruler proves that the king can't do this sort of thing. The nobles are prejudice against non-humans and against mages, not to mention the fact mages are legally forbidden from holding power is just reason enough.
As for Anora, they elect her. Similar to the Witnagemot, the Landsmeet mainly elects and confirms members of the Therin Bloodline. However, Anora can be elected queen. Just the same in the witenagemot mainly kept to the Cerdicing Dynasty, however a few times they branced out to people who married in like Anora did or from another House all together, ie Penda of Mercia.
My main issue is for the Dwarf Noble. Your origin is your coming of age rite goes wrong as you are rightfully or wrongly said to murder your older brother. You spend IMO the same amount of time in Orzammar doing stuff for the dwarves as you do for the humans in Denerim. A lot of the decisions have more impact in Orzammar for the dwarven character. However at the end you do not get the chance to rule. I would have been happy with just a simple opition. do you plan to follow traditions(ie be like harrowmount) or plan to carve a new path into the future(ie Bhalen)? this would have been fine and I would be happy, but nope no ruling orzamar for DNs.
The Dwarf Commoner on the other hand could never rule, he/she is casteless. On the other hand, your sister becomes the mistress of Bhalen and could give birth to a son. While it is not the same, it still is meaningful. I would have liked the opition to take the Carta over though 