I don't think you can blame Sid Bowditch for the design weaknesses of the twin rail machine, I believe he only designed the single deck version, which was quite good from the fatigue life point of view. To critique that design we'd need some pictures of cracked single decks.
The source of the problem with the twin rail design may have been vertical resonant vibration of the engine and rails as a mass-and-spring system. That is not a mistake Sid seems likely to have made, particularly if the rail resonance was the source of the excessive vibration so many twin-rail SB45 owners complain about. There must have been a reason for the complex construction of the single decks, and overcoming a resonance problem observed during product development may have been that reason. Of course if cracks developed the vibration would have increased in amplitude.
