We all know that Kana is one of those artists who live to please the audience. I think what she wanted to do was what she did when she debuted. Her main interests then was hip-hop, R&B, reggae and folk songs. But it's a hard industry, and you can't afford to just sing whatever you like. It was a harsh reality she didn't sell well, so she (and most people) need to adapt to please the audience, and the music industry works that way.
She had freedom to choose a-side songs after Tokutemo? I don't think so, but she had some freedom on b-sides, that's why they have more variety.
But that's how real world works, sometimes we need to adapt or else we are "extinct" (sorry for getting Darwin here ahah, I couldn't helped it).
I believe that right now, she enjoys what she does, and as long as she sells, she doesn't need to change her sound for something diferent like reggae or soul.