I agree with everything you say - except that only in quieter times was the Bex part of an Albula Expresszug. When it was run in two sections the 'B' (which left first) was certainly added to the back of an RhB train but in high summer I am sure I've seen a photo of the main portion (Bex-A) storming up the Albula with two Gems throwing diesel fumes to the heavens. It will take me a while to ;locate it but I was surprised to find it when I saw it too...
WRT the return trip, certainly by 1988 the C and D portions (C portion up to 3 coaches; D portion originally 8 but latterly normally 7), arrived at Pontresina at different times. These were added together and with a B or an AB (there were through coaches from Pont-Chur and vice-versa in those days), at the front and a restaurant car between that and the Bex, it was certainly hauled as a complete train back to Chur, normally by a Ge6/6ii. There are several photos of it.
While an RE with a Bex Tail is not unknown, when the main 8 coach Bex was hauled to an engine change at Samedan the front portion was normally only a Ge6/6ii, an AB and a B, so little more than through coaches Chur-St M.
It is certainly true, however, that there were many changes of stock and policy over the years; nothing stays static for long on the RhB - which is one of its charms ! (At least we can agree on that, I'm sure!)
Kind regards, Andy McMillan