I thought that Warner was the initiator of the scheme and he roped Bancroft in to doing the deed. Smiths crime was that he found out what was going on and did nothing to stop it. It is quite possible that the 4 bowlers knew that something was being done to the ball but assumed that it was legal and had no idea that sandpaper was involved. Bancrofts vague answers in the recent Guardian interview have restoked the controversy and led Michael Clarke to make comments that were completely speculative and not backed up with any evidence.
The whole episode will not go away because the investigation by Cricket Australia at the time was totally inadequate. Rather than uncover the truth CA wanted the whole thing to be sorted as soon as possible and they thought the lengthy bans would put the issue to bed . Wrong. A proper investigation should now be conducted by an external organisation who would interview every Australian Cricketer and staff member who was in South Africa at the time, including the CA people who conducted the original 'investigation' Only then will the real truth come out and Australian Cricket can move on properly.
It is clear that no matter how many first class runs Bancroft makes in the future, his international career is over, despite what Paine says. I would be dubious about Warners future as well given his obvious impact on team culture and Smith would not be my choice as captain when Paine retires.