I think that Alan did really have an important role in composing TR.
it's obvious that many elements of the song come from him.
many parts could have been composed only on piano.
and many arrangements that fans love from the 80-83 era are from Alan.intro and outro from R&J are the best examples, but there are many others. many fans love the Alchemy era, and if Alchemy sounds like it sounds it's also due to Alan, I mean the overal sounding of songs.
Now, Guy is the "musical director". In the 80ies, even if it wasn't official, it was Alan the musical director.
So don't forget that many arrangement we love (e.g. TOL in 83 and 85, PB in 83, bridge between Expresso love and DTTWW in 81, the ending of news, later of PI, angel of mercy in 81, the intro on Once upon a time, intro of WWE in 85, in the gallery and SHS in 80, etc etc etc...) come from Alan ideas. it's explained in the oldfield book that Alan had to rearrange all songs, while being on tour.
I don't want to underestimate Mark's talent, and I don't try to defend Alan because I don't like his behaviour, but his talent and what he brought to DS should not be underestimated either.