is there any indication that Herge even knew him then?
I've nothing at hand to say one way or another; however, someone has made the identification, so there may be specific evidence in the literature that says they did. In the mean time, I'm keeping an open mind, and will see if I can find further evidence to support a name for the figure.
If Hergé meant it to be them then I would think that they would have their faces turned towards the reader in order to be more identifiable.
It's probably missing the point a bit: virtually all the people he placed in that scene are turned away from us, but that's just the nature of this scene. Hergé included these figures mainly because he needed people to be in the scene, not because they had to be those specific
It's only because there are Tintinologists that the identities take on any real significance; their faces would have generally been unknown to the readership at large, even back in the day.
Hergé was always keen to get the attitudes of his characters correct and from life, and posed and drew whoever was to hand when he needed a referenece figure.
He himself would often pose for Tintin (there is a sequence of him doing just that in the documentary, Moi, Tintin
, which was used again in Tintin et Moi
There was obviously a degree of regard and affection in placing a person in the strip, but I think we shouldn't forget utility; it wasn't due to liking M. Cnapelinckx and the sandy-haired lad twice as much as Jacobs that put them in the frame twice, it was simply to fill four seats in the theatre he was drawing.