The next prosecutor would have to be extraordinarily stupid to get involved in this affair any further than required to put an official stop to the investigation.
In this case, however, I think Range actually wanted to be retired. He has taken so much fire over his apparently incompetent statements relating to NSA spying (among others he confused NSA and NASA on multiple occasions) that he probably wanted to get out of what is otherwise a fairly quiet job. In particular since there is pretty much nothing to gain anymore in this case.
I really hope that the investigation will be dropped officially. If it will mere be discontinued and put on ice, this would have a really bad psychological effect on Markus and André. Even if it is 99.999% sure that they won't face any repression, having the investigation dropped would be a clear cut, and enable them to be truly relieved.
In the US you'd need a declaratory judgment to deal with the threat of "legal action at some later point". But I think that's a civil law concept -- I have no idea how that would go in criminal law.