The whole scene with Buming founding out the secret plans is basically foreshadowing of what is going to happening later on the show which is Operational Stardust aka: drop the colony to earth. That entire scene was trying to build the suspense to the audience as Buming know the entire truth of what is going to happened in the entire team but got kill off, basically setting the stage of what happened next in the show. If he did survive or the incident didn't happened, Operational Stardust might be able to stop at it's track. That's what I think that entire scene was trying to tell. Also, Buming death suck as he is one of an interesting character in the show with some dimensional of character development.
For Nina issue, what I don't get is she saw Gato face in the 1st episode in the show which mean she should know that her former boyfriend was in the fight against the Federation. This make it worst of what will happened between her and Kou. If feel like the writer need some tension with Kou and Nina love relationship so, they shoehorn Gato former love interested in it. To me, if they alter this say, Nina saw her former boyfriend in the 1st episode and maybe make it in a way Nina talk about her boyfriend and what he did in the past to Kou in the beginning and make some sort of tension where Kou already know about the love interested much earlier and maybe use it as a weapon to proof how good he was again Gato and maybe win Nina love as the show process, then it might have a chance to make the relationship with Nina and Kou more workable. Unfortunately, what we got is basically what was shown in the show.
For Cima, the reasoning of betraying Zeon for Federation was not shown in the show which is true but you might get a hint of why she hate the Zeon from an extra is include in the DVD set. It kinda hinted of one reason of which why she change side. Go to the link below to see it.
https://www.youtube.com/watch?v=r6Qe4qnPFzc