What would constitute clear evidence of murder?
Good question.
In some cases I've followed, circumstantial evidence was enough to get a conviction for murder....
Even with all the twists and turns as there are here, with differing jurisdictions and charges going towards individual cases, and parsing out of potential evidence on multiple potential cases on a potential suspect at large going towards only a specific case...
Though it's awful and frustrating for this to happen in the case of MM, RIP dear one, I wonder if and hope beyond hope that the "German suspect" the prosecutorial and LE systems amongst the different countries may be ready to identify as a suspect in MM's disappearance/abduction/murder, Have a Plan for charging him toot suite.
As soon as he is released and any evidence that was used as part of another case with admitted crossover between cases (e.g. box factory images/evidence thereof or elsewhere related to the charges for which he served time in prison for just now) can and will be used to arrest him anew on separate charges...
Based on circumstantial evidence that's "in there" for other cases he is implicated in or has been looked at or charged in connection to, which is telling enough to lead LE in Portugal or Germany or elsewhere (e.g., a wider international crime ["trafficking"] syndicate operating across multiple countries, so Interpol perhaps?) for them collectively to bring down the hammer on him and cuff him anew.
Just hopeful and hoping not to be frustrated and disgusted that he may be allowed to waltz off unbidden after he is released after serving time on another charge he was convicted for molesting another young innocent girl, with no follow up on him being an official suspect and charged for Madeleine's murder just because the circumstantial evidence, if it was (e.g., box factory images), was already used to charge and convict him for his other crimes.
I just can't imagine that evidence which is incriminatory wholesale and indicitive of his depravity and criminality, which could include him possessing images of MM, would be prevented from use in this case.
Cuz AFAIK, there ain't no such thing as double jeopardy when it comes to using a trove of images criminal in nature to charge a felon for other crimes.
JMHO