Does anybody know if these details were already there in the original Belgian versions, and were simply used opportunistically by LLC and MT during translation, or if they were added in the first English edition to add to the effect?
I'm pretty sure that the moustache motif, which as you say is worked into everything in Borduria (including the car radiator grilles) is there in Hergé's original version - as a satire on the iconic nature of dictators' moustaches - and simply inspired LLC and MT to extend the joke with their name for the dictator. I've not read that anything in this book was redrawn for the UK publication (unlike the complete re-draw of The Black Island, for instance), though now someone may correct me on that!
maybe this is really obvious to everyone, but I was surprised when I suddenly noticed them just this last year!
It certainly wasn't obvious to me as a child reader, but it's always great when you notice things like that in a Tintin book you've been reading for years, especially when you spot it for yourself, rather than just discovering it from a book about Tintin. (I can't actually remember whether I spotted this moustache-motif one for myself or not!)
(PS: In using the British English spelling of moustache in my post, absolutely no criticism or correction is intended of your equally valid mustache spelling!)