IMO I think people confuse fandom Roxas with canon Roxas. Add fandom Ventus to the mix and people automatically consider Roxas the emo/grumpy one. Roxas is more down to earth, but he has an earnest and naive side as well. As someone who's a Days fan, I can tell you that Roxas did the whole childlike wonder before Ventus(Not counting Sora). If you play or watch cutscenes of the game(ignore the abridged movie) you'll see those qualities. Roxas gets excited about discovering new things like he did in Halloween Town and Beasts' Castle. He shows his childish/naive side in Wonderland and Halloween Town when he gets irritated by the Cheshire Cat and tricked by Lock, Shock and Barrel.
Unfortunately a lot of fans simplified Roxas to the ansgty version of Sora, when in reality he's just a poor boy who has been dealt a very bad hand. Of course Roxas behaved the way he did, he kept getting pushed into corners. Other than that, Roxas is a sweet, caring, naive and kid at heart. I've rambled too long...
While I agree with all of the points you made, it's also worthy to note that Riku is considered a grumpy and emo kid as well when he was under pressure from Maleficent/Xehanort but whenever he is with Mickey or Sora he turns back into a playful teenager, and his appearances in 3D and 3 are much more laid back and calm similar to Aqua.
And Roxas is a complicated case because the only game where we got to actually experience him fully without being under direct manipulation (interms of memory/personality) by others was Days who is probably going to be stuck to the DS forever, as more than half of the game was cut from the movie version like you said.
I personally view current Roxas (after everything he's gone through) as a more mature kid who has gone through a lot and just wants to stay with his friends. He's not quite as laidback as Riku or Aqua, but he's also not as numb as Xion or Isa. But we'll have to wait until Remind to see more.
As for the thread itself, I think the only proper functional trios are the Destiny Islands and Wayfinder trios, like SweetYetSalty said. While the narrative for days ties to put Axel, Roxas and Xion into a trio, they don't actually function as one: Axel and Xion's interactions are weird and out of place for the most time and while they did end up caring and loving each other, it's clear that they're only really there because of Roxas. To add to that, the other two trios are mostly singular; while they interact with other characters, their arcs revolve around each other at all times.
While for Axel, Roxas and Xion they don't get that same sense of belonging while also being influenced by external sources (Axel has Saix, after all. Roxas and Xion both have a complicated relationship with Naminé, and Xion with Riku too. Roxas later gets Hayner, Pence and Olette as his friends too, even if their friendship is questionable at best they are associated with him before we even got to meet Xion or see Axel and Roxas' friendship in proper).
What I meant to say in this ramble is that while they serve as a trio of protagonists for the narrative, I don't really think they were intended to mirror the other two trios in any way or form which is evidenced by the finale of KH3 where Isa joins them.
