Well, naturally, Sora has a strong heart as evidenced by his ability to wield the keyblade. Since Sora didn't spend too much time as a heartless, perhaps less of Sora's heart was able to fall into the darkness, thus Roxas retained some small portion of Sora's heart. As Axel interacted with Roxas, perhaps the influence of Sora's heart made Axel feel like he had one too. Unfortunately, that is a complete theory, so make up your own.
KH2FM has its own forum, so go look there. However, I will tell you that Square Enix has no plans to bring Final Mix to America. They didn't bring the original FM, but perhaps due to high demand, they might release it to earn more money.