This feels like a matter we both will be on a impasse over, so we’ll just have to agree to disagree.
But I do wish to point this out:
It is true one can’t really choose the type of family they were born into. But they CAN choose the people who they truly feel a kinship and how little interaction they will have with their blood related family, especially when the family environment comes off as toxic. And let’s face it, nothing about either of the Second Prince’s parents inspire a sense of a healthy family home, and the Prince has every right to limit his interactions with them as much as he sees fit (especially as he gets older and becomes less reliant on either of them).
No one should have to suffer being around a unhealthy family environment, especially when the most toxic family members clearly have very little desire to change their ways for the better.
It’s really is all up to you whether you want to make another story; you are the author after all.
But I’d personally advise against it (especially if this is the first time you’ve decided to create a lengthy story); there’s often a strong chance of running yourself so ragged trying to come up with ideas for more than one story that none of them end up being finished or their quality actually lessens due to. It may be best to focus on Kingdoms for the time being (though you could also privately work lightly on your other story during times where you are in a slump with Kingdoms).
Yeah, Arlan’s still going to call him out for being too afraid to move on once he learns the full story. I mean, Arlan had Melissa forcefully taken away from him by the King while also being forced to marry a princess who he feels has every right to hate him currently and pretty much grew up with emotionally distant father that despises him (as much as I like Javier and know that he’s one of the King’s personal knights, i really doubt his words about Andus being proud of the Second Prince) and a mother whose bitterness and ambitions make her a little similar to the King himself (though instead with some affections for her birth son).
But despite his care for Melissa, it won’t stop him from still wishing to (at the very) to least be friends with her again and hoping that she can find happiness in her decision to follow her duty in some way.
Despite being forcefully tied to a marriage with a girl he barely knows, it won’t stop him from trying his hardest to understand Sophia and at least work on creating a friendship to the point she knows that he is someone who she can rely on with as much faith as Clithus (still not sure if she would eventually develop feelings for him that are platonic or romantic; gotta see more of their interactions).
Despite being the son of a King who turned his heart cold and a mother shows just as much favoritism toward him as the King shows toward Marxus, it won’t stop him from caring for Marxus and wanting to help his brother be a good king with the skills he has to offer.
Arlan may have had to deal with a good deal of hardship as a child (and knowing Andus currently, he’s only going to try making life even more crappy for his less favored son), but it doesn’t stop his care for others, his determination to grow stronger for himself more than anyone else, and his will to move forward embracing both the good and bad that life will throw at him.
Well, that doesn’t narrow the list of ROs at all; good job 
(no really good job; it’s a tantalizing tease that’s ambiguous enough that it leaves a person wondering ‘who is the Prince embracing’).
Part of me is betting Sophia due to the fact the kiss being ‘slow and soft’, giving me a bit of the impression that the embrace is being slow and careful as to not make the partner nervous, along with the fact that the declaration of love is whispered, as if the lover wishes to keep the moment private between them (Sophia strikes me as someone who enjoys a good deal of privacy when showing her affections and care for someone, especially when one considers their relationship between their homelands currently). Also I like the idea of Sophia getting a little passionate and enjoying the affection so much that she decides to pull the Prince closer to her. 
But considering that you’ve described Clthius as a protective lover, I also wonder if it is her. Being a knight means she’s clearly able to hold her own in a fight, but being a protective lover give me the impression that while she’s likely passionate, she’s also really gentle with her lover (especially if they are not as strong as her). The fact that the embrace is considered ‘comforting in ways words would never be’ sounds like the partner is someone who mostly relies on their actions to get across their true feelings (something I could see a action-oriented knight preferring to do rather than just relying on words). There’s also the fact the partner pulls the Prince closer; aside from the action coming across as a little passionate, I could see someone who is quite protective of their lover wanting to be as physically close to them as possible, because the closeness reaffirms that the person they care for is in their arms and safe. 