All the Tears that Shine was indeed sung by Mike Sherwood, who also sang a few tunes on Steve Porcaro's solo album.
The Sherwoods (Mike and Billy) go way back with Paich and Steve Porcaro, who produced an album for them back in 1985/86 (Nomadic Sands by Lodgic). I think Mike Sherwood sang backgrounds on Lea off of the Fahrenheit record, recorded right around that time. Lodgic also comprised Jimmy Haun and Guy Allison - Haun would later work on Yes' Union album alongside Steve Porcaro and Allison joined the group World Trade, which also featured Billy Shrerwood and Joseph Williams' brother Mark Towner. Billy Sherwood and Mark T. Williams then toured with Steve Porcaro in Chris Squire's band in 1992, when Billy Sherwood also co-wrote The Other Side with Paich for the Kingdom of Desire album. Right around that time Steve Lukather hooked up with engineer Tom Fletcher, who had worked with the Sherwoods since their early days in Las Vegas, and had him co-produce Candyman and FOH engineer the subsequent tour.
Mike Sherwood recorded and toured with Air Supply in the early 1990s (Mark T. Williams joined Air Supply cirka 1993), before releasing a solo album, Tangletown, in 1998:
https://michaelsherwood.bandcamp.com/album/tangletown. There are a few real good songs on there.
Unfortunately Mike Sherwood passed away a couple of years back, so including his demo vocals on his album was a kind of homage from Paich.