Samuragochi recently held a press conference.
This detailed summary clears up the situation further.
He admitted that, while he was a professional composer before, he hasn't composed any music at all since the 1997 film Cosmos which started his collaboration with ghostwriter Takashi Niigaki.
This confirms that Niigaki is the composer of all works since then, including Biohazard Symphony, Onimusha and Hiroshima.
Samuragochi said that he laid out a "detailed blueprint of the content" for the music and Niigaki then wrote the notes to it. So Samuragochi apparently had the visions and Niigaki realized them.