Just read an interview of Wolters in the Focus magazine (2 days old, German), and it looks like they hope to build a case in the next months, before he is released in early 2021 (hoping he won‘t be released early in this situation). They are apparently sure CB did it, but hope for more hints to have a case they can prosecute. It is still possible that they will not be able to prosecute, without additional information. And they hope they can tie him to other cases...
The bottom line seems to be that they hope to find enough evidence on any case, so he won‘t get out ever again, with a ‚Sicherheitverwahrung‘ following any release from prison. (Basically keeping him locked up even after he has completed his sentence, because he is too dangerous to release). Their biggest fear seems to be that he ever gets out again.
After that interview I am not hopeful he will be locked up or even tried for MM. On the other hand, I wonder, if the sentence for the rape of the 72 year old American together with a conviction for the Irish woman (same modus operandi, she might recognize his voice...) couldn‘t be enough to lock CB away for good. From a distance this looks to me like the most promising path...