Can confirm that all UA numbers operated as United Express (SkyWest, Atlantic Coast, etc) work fine with Diamond Club. And with a minimum of 800 miles on non-BD sectors, those 30 minute feeder hops to IAD or LAX for long-haul connections can prove pretty good value for money, too

ACA is breaking free from its agreement with UA, but details - dates, etc - are yet to be finalised, I believe. Presumably UA mainline or US shuttle services will fill the service gaps on the east coast.