I would be extremely hesitant to compare intricacies of ECN to C41. Using couplers that require more silver for a given density is of far more concern for motion than it is for still photography. it's absolutely an economies of scale issue at that point.
Vision3 500T uses 5.5g of Ag per square meter. whilst I don't have data for a c41 stock I would not be surprised if on average they used more silver to achieve the intended purpose. Different dye sets, different colour developer. Just because it's ideal to use less silver if possible doesn't mean they can. stock to stock silver amounts vary wildy for far more reasons than just couplers utilised.
You're correct in pointing out the issue of dyes being converted back to a leuco state in bleach being fixed, but it absolutely doesn't mean the opposite can't still be an issue.
I'd be far more inclined to follow the words of the man who invented C-41 blix and hated it rather than some citing that seems largely unrelated.
Point is, I wouldn't advise the use of blix because of leuco dye risk, silver retention risk and overall chemical instability and resulting shelf life. Labs check for silver retention once or twice daily and they don't use blix! if kodak supplies bleach and fix to labs there is probably multiple reasons for it.
Risk doesn't mean certainty, but why involve risk at all if its simple to avoid?
