You don't think Coleman would have taken the opportunity to split the workload between 2 high quality looseheads? Obviously Bell was on a serious trajectory but even now his scrummaging is still a work in progress. At the time HJH (Harry Johnson-Holmes) (Harry Johnson-Holmes) switched he was clearly the better scrummager of the two. Accept that HJH (Harry Johnson-Holmes) (Harry Johnson-Holmes) might have had a financial incentive to switch as well but he's moved from being in the top 3 or 4 looseheads in the country to well down the pecking order at tighthead.
Not sure Sio fell too far down the pecking order, he basically slipped to 3rd choice behind Bell and Slipper and that was partly due to him seemingly playing injured (until this year when he seemed fit and powerful again, as noted of course by BR but also many others in this forum).
Well no, I think Coleman would also prefer to have his two best props on the field for the majority of the game by playing Johnson-Holmes at tight head.
That is correct that Johnson-Holmes was a better scrummager initially but it was also very clear that Bell was going to be an incredible player very quickly. His trajectory was always very steep. Johnson-Holmes played tight head in the under 20s. He moved to LHP initially because that is where there was opportunity at the Tahs.
Johnson-Holmes has had some inopportune injuries. I still think he's in a similar space. If he can put together a strong Super Rugby season I think he's every chance of going to the RWC. His play around the field and ability to play both sides of the scrum make him a good option. At THP I think it's pretty open after Ala'alatoa and Tupou and there's every chance Tupou can't go to the RWC.
If you looked at the Brumbies situation and they didn't have Ala'alatoa at THP and instead had someone pretty average, I think it would have been highly likely that Slipper switched sides at Super Rugby level. It just doesn't make sense to have one of your clearly best two props on the bench for most of the game.