Good news about 4, 5, & 6
I think Ronaldo has to be guilty of the first two murders. If he's innocent, that would mean that Laure has screwed up again & will surely get the sack (which can't happen, as the series has been renewed).
So, if Ronaldo killed the first two, who killed the third? The guy who chopped up his flatmate? Ronaldo's mother? And how did they know of the scalpel-burn signature?
I'm also wondering what the significance is of the prostitutes. We didn't see the first two victims before they died, nor see any of their associates... Yet we saw victim number three before she died (Tatiana?), along with her fellow prostitutes, and are still seeing her fellow prostitutes and pimp. Something more to it? It must surely be linked in some way...