Here's the email address for the museum: info[at]museeherge.com. I expect that they have English-speakers among their staff. If you specify "Ce message est en anglais" at the top of your email they might get the message.
Here's a
short bio of Herge which is apparently based on interviews he had with Numa Sadoul who published a famous book based on them. It includes a photo which purports to be of Herge as the head of the Squirrels patrol.
Scroll down to about 1931-32 and you'll find what appears to be a photo of Dedonker as Tintin returning from the Congo.
According to the opening article of the link you have provided, Herge was forced to leave the scouts for the Federation des scouts catholiques de Belgique ("Federation of Catholic Belgian Scouts") when he left secondary school - something he took very bitterly though it provided him with the opportunity to publish his drawings.
There's nothing much in terms of scouting in the Peppermans article, based on his attendance of a Friends of Herge gathering, just that he was told by his scout leader to report to a Mister Herge, a name that meant nothing to him. A few days later he was travelling to the Gare du Nord in Brussels with a very nice man (Herge) who helped him dress up as Tintin and told him not to worry and that everything would be fine since he was expecting just a handful of readers. Peppermans was supposed to deliver a speech but they were both quite unprepared for the 3000 people who turned up! and as a result he soon found that the speech was of little importance since the crowd simply wanted to see Tintin and his creator.
Of course, until we know for sure what happened to Peppermans, and that the old man at the gathering was the genuine article, we are going to have to take these revelations with a pinch of salt.
Good luck.