How does any of this even make sense? If it's a politically arranged marriage, how can you just arbitrarily substitute just anyone for the position? Especially someone who hasn't even been officially be recognized as being of the Royal lineage in the first place? What advantage would it gain your kingdom to substitute the heir to the throne for a random daughter of a concubine that no one in the kingdom even knows exists?
This entire story has a paper thin plot, but this is even more unbelievable than a teacher in your school hiring thugs and assassins to kill people, putting one of the Kings daughters in danger, yet not even getting scolded or punished for the transgression. More unbelievable than having the Principal, and possibly most powerful person in the series so far, be completely oblivious to a student in his school running off to defeat assassins while he was in the office with him describing the aforementioned assassins. I mean how is it that Sirius manages to arrive at the scene of the crime, but neither of the teachers even show up?
I'm even willing to overlook that a 12 year old barely into his second year at magic school is preparing himself to challenge an elf that's hundreds of years old to a battle because he thinks there might be some retribution for saving his students from assassins. This "swap up the arranged marriage to the well known princess" thing is just a step too far.