Regarding whether I think of it as a solo album, I at least think that Layla and the “solo album” that came out before it are part of the same project. Duane Allman wasn’t a touring member and they played live as Derek and the Dominoes before working with him in the studio. They were beginning a tour in support of the 1970 album that’s just called “Clapton” which all the Dominoes played on. If Layla isn’t a solo album then Clapton should probably be considered a Dominoes album.