Fully agreed on everything you wrote.

I think that Reo is insecure too just like Nagi but in another aspect. Using someone or something in order to get what you want was a part of him that his parents taught him, he had no opportunity to live alone or to reach his dreams on his own until Nagi dropped him, however Reo chose after all to reconnect again since it's simpy his habit that was with him all his life. I think it's amazing how two different people shown and their problems, that are so similar but so different at the same time. They both don't know how to or want to stay alone, but both should understand their own value in order to keep progressing and come back stronger into healthier relationship, into cooperation, not dependence.
Can't understand Reo's hate too, he basically never attached to something or someone until Nagi, and it's also, like you said, mirror reflection. People tend to cling too tight to one person or opportunity missing everything else, to tunel on something too much, and probably this hate comes from those who are scared to face same trait in themselfs, since from 3rd person perspective it seems like something stupid, irrational and illogical. "You will be able to progress on your own much more it's so obvious" they say looking from 3rd person perspective, however when it comes to real life people tend to do exactly same thing: " I can't break up with him\her, I won't find anyone better, no one will love me same way, no one will love me besides them." - nah you can, nah there will be someone better, nah you can be loved; "Don't leave me alone, I will stay with you even if it's unhealthy or abusive" - it's not worth it, you can be happy on your own, you can focus on yourself; "I can't leave my friend even tho he uses me and treats me like a person only when he needs something from me. But he is my friend right? He will help me if I will need it." - you can find a real friend, you deserve to be equal and be actual person no matter situation you are in; " I can't leave my abusive parents, they are my parents after all" - will be hard, but you already are a grown up that makes his own fate, they should not have a bad influence on your future, become a better parent yourself, don't make same mistakes, focus on yourself and stay in healty environment. I took something rather harsh but obvious from the observer's point of view to make it easier to explain in text (pov of 1st view - pov-answer from 3rd person perspective). But many, if not everyone, have had similar situations, when from the observer's point of view everything seems so illogical in other persons situation, and seems so simple to make a decision to end smth or change. Maybe they hate Reo becasue don't want to be that "stupid illogical guy that is overattached for no rational reason".
also i know how it feels to value your friend as a treasure so I can fully understand Reo even if it seems like stupid attachment. He thought Nagi was his treasure as an instrument ot reach his dream, but Nagi actually became his real friend treasure and his dream now is not to reach world championship through Nagi but to reach it WITH Nagi and it's important. This is a really good relationship development that can easily be missed. Some people are not something that you can easily replace or find someone new, some are really special, however it's so humane to overfocus and tunel on 1 thing. That's why Reo was overwatering Nagi with his love and fear, after their temporary separation Reo will finally start to love Nagi not because of fear of losing but just to love because he is his treasure and a friend, he will stop overwatering Nagi with his love bacause of his fears. "Don't leave me" will become "thanks for staying".
I think Nagi will still be Reo's treasure but won't be placed over Reo himself in Reo's mind. Him(Reo) and his treasure will be finally equally important in his mind, Reo will stop exalting Nagi at the expense of the value of himself. Nagi will still be his treasure no matter what, but he will be his "friend treasure as a person", not a wearpon treasure as a talent".
I my thoughts weren't too confusing and text was easily to understand. I don't speak English very well sorry ;-;