inspiration for them came from "a picture of two mustachioed, bowler-hatted, formally dressed detectives who were featured on the cover of the Le Miroir
That's the story (sort of) given in Michael Farr's Complete Companion
, although in that instance he says only that it is "probably" the inspiration.
I think that elsewhere it's been said that the magazine only became known to Hergé very much after the fact, and that as ever, he maintained that the inspiration rested with his father and his uncle.
It's possible that "Milou" was the nickname of a girlfriend
That's actually quite definite, it really was the nickname of Marie-Louise Van Cutsem.
However, the Cottin nickname thing is at the very least an interesting coincidence, so it's a good spot, and suggests that it was a name-form in some degree of common use, rather like Tintin
being a name which circulated as a nickname over and above our boy reporter.
Cottin was "Louis Émile", so I would wager that in his case the nickname was a portmanteau formed from the start of "Lou
is" and the end of "Émile
", reversed in the manner of Hergé's own initials forming his pseudonym.
a new angle on one of fiction's most sweet and innocent doggies
It's only the good angel on his right shoulder keeping him from a life of whisky and anarchy, eh...? ;-)
Since I first heard about her, I've always thought it interesting that Marie-Louise became Milou
, rather than Malou
; it may be in spite of, or because (I don't know which) that just round the corner from Hergé's family home in Rue Philippe Baucq is the Avenue Jules Malou
, named after a somewhat controversial former Prime Minister, who caused rioting in his day by undoing the laws which had made the Belgian education system secular, allowing the Catholic church to gain influence - perhaps the Van Cutsem family didn't look favourably on his legacy?
In re: your point
I wonder if the name was mentioned in the reports following Cottin's arrest, trial and conviction
It's not a definite indicator of currency, but I think the fact that I could come across this local American paper, The Woodville Republican
, of March the 1st, 1919 (I think that we can sidestep the gross typo in the dateline, which renders the month as "Mcarh"!) - which not only carries the story of the attempt on Clemenceau's life (if we learn only one thing from this, it's that standing still to tell everyone that the first shot missed is a mistake if someone is trying to shoot you, as the next half dozen or so might hit you...), but also that his nickname is indeed Milou.
If local papers abroad had that detail, then most likely the papers in Europe would have had this too.