On a side note, on the 3-in-1's, the galleries are mirror images. (To the Methuen hardbacks anyway) The front left hand gallery is the final gallery in reverse, front right becomes front left. This is repeated on both end galleries also. (Phew, I hope you followed that!) ;-)
I've just noticed that my old Methuen hardback copy of The Broken Ear (a late 70s or early eighties edition, I think) has its endpapers a different way round from my 80s copy of The Blue Lotus. I'm not sure which is the standard order, if there was one. (I'll need to go back upstairs to look at my other hardback copies.)
Is it just the page order which changes, or are the images literally in mirror-fashion?
I can't answer for Ranko's copies, but they're not flipped into mirror images in my books, just re-ordered. Flipping drawings is a big no-no for most illustrators, although publishers sometimes do it, to make a drawing fit a publicity purpose. Apart from being generally wrong artistically, it would put everyone's buttons, swords, etc, on the wrong side!
Decorative endpapers were a feature of many books of the time
And indeed still are, of course, especially in children's picture books.
Were they something used by the publisher to promote the books by making the reader curious or were they something thought of by Hergé?
Whether it was his idea it was to update the endpapers and to have this portrait gallery, or his publisher's, Hergé certainly seems to have thrown himself into the task with enthusiasm. There's so much thought gone into each expression and each picture frame style, and looking at these endpapers closely following your post, I'm struck, not for the first time, just how beautiful the line drawing is - right up there with Hergé's very best.