If Alan didn't "compose" the piano intro for Romeo & Juliet, do you think he would have been "allowed" to play it as intro for Layla on EC's tour ?
On the oldifield book (a book that has been approved by the band and MK) it is written that during the OL tour, Alan had to re-arrange old songs, and to arrange new songs with only one keyboard player, while studio versions contains several keyboard parts. And of course "building" Telegraph road.
Of course the line between "arrangements" and "composition" is tiny, so it's always difficult to determine if a musician had a "big" part in a song or not.
"Money" or "another brick in the wall part 2" are credited only by Waters. but on both songs, the guitar solos are not only "solos", imho, they are "melodies" of the song that almost all people who know the songs can sing. Should we consider these parts as "composed" elements, or only musicians "contributions" ? I must admit I don't have the answer.
Is a song only a melody and a chord progression ? or should we take all arangements (tempos, rhythm pattern, sounds, instruments...) as "parts" of the composition ?. I don't think the answer can be always clear and simple
I think that if you name the song "baker street", you will immediately hear the saxophone theme. But was the saxophonist credited for "composing" the song ? I don't think so. Should he have been credited ?
I think Alan had a "big" part. I don't know how much it can be named as "contribution" or "composition"
Funnily, when the person involved is famous, then all come with lawerrs : Sting is credited for MFN, just for a melody line, but as Dusty said, maybe that Guy had a bigger role in the song...
another example : songs credited by lennon-McCartney... we all know that after 65-66, it wasn't the case. and "revolution 9" was done by John, Yoko and George, but is credited to Lennon-McCartney !
so yes, I think we will never know for sure