Two things here...
Great call on Sandelin- he's been a great coach at UMD. Same questions about translating to the NHL though. I'd throw the name of Greg Carvel out there as someone who could make the switch...he's obviously a great coach, but he has 7 years of experience on an NHL bench and at 51 is a good age to slide in. I hope to god he doesn't though.
Regarding the bolded. I'm just spitballing, but what if Leach did a good job with those young guys and it was on Cassidy? Someone made a list on here of guys that came up from Providence and almost all of them showed flashes at the NHL level before getting stuck in Butch's doghouse. Now, there could be a lot of reasons- maybe they weren't NHL ready; maybe the lines of communication between Providence and Boston were poor about what skills to develop; maybe (as you note) they just weren't good enough; maybe Butch was too hard on the kids; maybe Leach is bad at player development; who knows.
But it did occur to me that a lot of those guys showed flashes before getting shut out, not sure where the blame lies on that one (although I think we know where the FO thinks the blame lies)