Warren Haynes did write "Soulshine", he was 34 when "Where It All Begins" came out. He is 13 years younger than Gregg Allman, but he plays with them quite frequently.
The singer you refer to in the duet with Warren Haynes on the Live At Bonnaroo recording is South African vocalist Vusi Mahlasela - Mahlasela has never performed with Simply Red.
The Allman Brothers wrote that song, but Warren sang it with them and without them. It's the most beautiful song in the world!!!It really does touch your soul, so a they say, listen what your daddy says!!! Warren is so much younger than the ABB that he could never have written it!!!