I keep thinking that what I learned decades ago is that rear end width (no, not the part that's relevant to saddle selection!) is not a factor in chainline for a road bike. The chainline measurement is a distance from the center plane of the frame to the right. At that distance you should find the midpoint of the group of chainring planes and the midpoint of the rear sprocket cluster planes. When we went to 6-speed we added 6 mm total to the rear spacing, 3 mm on each side. The drive-side flange moved 3 mm closer to the frame center plane relative to 5-speed. The 6 mm-larger width of the cluster is still centered at the chainline distance from the frame center plane. Hence chainline is not changed between a 120 OLD and a 126 OLD, and the BB width for the same crankset is not different between a 120mm and 126mm frame.

What did change is that the bracing angle for the drive-side flange is less than it was with 120, and hence the potential strength of the rear wheel is reduced. But that is not affected by how far the crank sets from the frame center plane.

I don't know why for the early models, Campy had two spindles for 110 and 120, unless they correlated to simply a much narrower tread (Q) for single (pista) drivetrains versus 5x2.

Rear spacing is certainly a variable in frame/drivetrain geometry, but I don't think it is a necessary consideration in setting up Campy spindle BBs, beyond the early 110/120 distinction.