That's interesting that they kept the VA private. My personal opinion is that it is indeed Watanabe and they kept that hidden in order to not spoil any of the story's undertones.
The Klonoa fan disc series was created by a circle dedicated to video game fan discs, which is a common practice in Japanese fan culture. I can only assume the vocalist was another fan involved with the project. It's very rare for people who worked on these franchises to get involved, and the only case for Klonoa was when composer Eriko Imura helped with some piano solos in a separate independent music project.
There is a karaoke sheet for Stepping Wind which is why it's so easy for fans to create covers in Phantomilian. I forget where exactly it came from, but I'm positive it's in some official material.