Ok, some paragraphs would have been nice since it's a bit hard to read, but you got several things right here. ^__^
We all know Roxas and Ventus look alike and can use the Keyblade and have connections to Sora. Ventus' heart residing inside Sora's might be the reason Roxas looks like Ventus kinda like how Vanitas looks like Sora because of how Sora and Ventus' heart connected on Destiny Islands.
I got to thinking though maybe there is another reason. Maybe when Sora turned the Keyblade of People's Hearts on to himself to release Kairi's heart it also released his and Ventus'.
Kairi and Ventus had hearts of pure light and couldn't make a Heartless which is why they went to Sora.
When Sora released the hearts, Kairi's went back to her body and his own heart became a Heartless. What happened to Ventus' heart? It followed to where Nobodies are formed, Twilight Town, and joined with Roxas because it could feel Sora and it didn't sense darkness like it did with Sora's Heartless.
That Ventus' heart being inside Sora is the reason why Roxas looks like his identical twin was actually confirmed in the Birth By Sleep Ultimania.
It is however not the same as with Vanitas and Sora because Vanitas got his looks from
his connection to
Ventus who was in turn connected to Sora, so Vanitas got his looks over Ven. It was also confirmed that had anyone else helped Ventus with the first injury of his heart, Vanitas would look like that person. So if Riku had been the one to help Ven Vanitas would have looked like Riku.
Roxas however got his looks
directly from Ven's heart, not over the connection with a third person.
When Sora used the dark Keyblade on himself he only released Kairi's and his own heart. Ven's heart is crippled and inactive so it remained in the shell Sora left behind, his original body. In that process Roxas was born and inhabited Sora's old body, which got then shaped to look like Ventus because his heart was the only one left inside the body.
Ven's heart could not "follow" anything or consciously "join" with anything because it's asleep due to its crippled status.
Basically, Roxas was a Nobody that always had a heart which is why he had feeling all throughout Days. Since he had a heart, that means he wasn't a Nobody, but he still was because he was Sora's Nobody with someone else's heart, Ventus'. This is the reason he had emotions, one of the reasons he could use the Keyblade, one of the reasons he passed out in Castle Oblivion, and the reason why he looked like Ventus. The last thing I would like to explain is myself saying Ventus had a heart of pure light. When he defeated Vanitas and destroyed the X-Blade, they unknowingly separated again which is one of the reasons he fell into a comatose state. This is also why Vanitas appeared with Young Xehanort in La Cite de Cloches. YX is hosting Vanitas heart and Sora is holding Ventus'. Vanitas is pure darkness and Ventus is pure light. That is why Ventus couldn't become a Heartless and why his heart found shelter in Sora's heart until he joined with Roxas. I hope evryone understands this because I believe this is a little jumbled.
Yes, Roxas always had a heart, but he couldn't feel real emotions right from the beginning, as seen in the beginning of Days when he acted like a Zombie because he was totally blank. Ven's heart was there, but couldn't be really used properly by Roxas.
As we learn in DDD Roxas most likely developed an own heart alongside Ven's, so how much real influence Ven's heart had beside giving Roxas his looks, the ability to dual-wield (which was also sealed at first, well, because Ven's heart is sealed too due to its state) is now subject to debate.
I would assume though that Ven's heart being there might have had an impact on how Roxas' own heart was grown and shaped, and the speed at which it happened as Roxas was clearly the Nobody with the most real emotions and who exhibited them the earliest.
With DDD we know now that Vanitas still exists in some form, yet Ventus falling comatose was because his heart was utterly destroyed by the destruction of the X-blade.
Before their Final Fight Vanitas warned Ven that his heart had become a part of the x-blade too, and destroying it would mean the destruction of his heart.
Ventus however went through with it anyways in order to help Aqua and Terra and also because he didn't want to be part of the X-blade, he never wanted to fuse with Vanitas so its entirely possible that they didn't separate unknowingly, but rather willingly at least on Ven's part.
If Young Xehanort somehow really holds Vanitas' heart or not isn't confirmed, the only things Nomura had to say on the matter is that Vanitas, unlike Ansem SoD and Xemnas, has no physical form right now and that he appeared beside Young Xehanort because he reacted to Ventus inside Sora.
That Ventus couldn't, or rather didn't become a Heartless is 1st because his heart is crippled and 2nd losing your heart does not automatically mean you become a Heartless. A heart only becomes a Heartless if its consumed by Darkness or captured by other Heartless.
With Ventus it was neither, he lost his heart due to severe injuries after destroying the X-blade, not because it was consumed by Darkness or captured by Heartless.
Riku also lost his heart in KH 1 when Ansem SoD kicked it out of its body, and he also didn't become a Heartless.
