If "Catcher In The Rye" makes the cut, Brian May will be on it. I don't think that they would use a different version where he isn't featured on. But I really doubt that the track will be on the album. Maybe as a bonus track on the first single or whatever.
or is it the case that BBF and Fortus will take turns to share duties with Finck on every track?
I don't think that they will share duties. I think they will be doing the tracks together.
For example "Iron Maiden" were doing it too with three guitar players without any problem.