@diogenes i think its a good enough reason, he practically went: ''fuck what my dad the king decided about us and fuck all the official procedures to do this properly, i'll cancel our engagement!'', causing a bigass scene in a major social event and akari almost fucked up the relationship between the two countries, also someone who is constantly in the public eye like a prince just showed that he can be easily manipulated, that in top of him throwing accusations without properly investigating them, at his fiancee, a girl that doesn't even have bad rumors about her, her only defect being too naggy for him, nah mate he royally fucked up.
Since he's the 1st prince im guessing a younger brother will be the heir now, and other nobles will probably try to form a faction around him like u said since they will also think he's easy to manipulate, but honestly they would still need him to be popular enough for a chance of being king again, and with this royal fuckup he probably wont ever be able to recover, 16 in their world is a full ass adult so no excuses for his age there, the only good thing is that he realizes he fucked up and matured a bit thanks to that so he wont be that easily manipulated in the future