No matter what an expert's study on the case might look like, the proof of availability for a sufficient number of free licensed content to counter GEMA's presumption has to be brought forward to court. The "GEMA assumption" is part of German jurisdiction, therefore it's much likely the ruling has to be found at Germany's highest court, the Federal Court (BGH). But the true challenge is its verifiability.
Tag / authors
The licence market is protected by the GEMA presumption. The income of composers, authors and providers (labels) of GEMA licences is protected. Where's the threat? They are "threatened" by new providers, say authors of works under Creative Commons licence. Those are constrained in selling their licenses. Free licensed songs would represent a product substantially expanded by valuable properties. It is no substitute product but nevertheless a competitive one.
The line between private use and commercial use is not that clear, and it is not explicitly defined by Creative Commons. The definition rather relies on the author being the licenser. But in case you are about to license a work - just ask the author if the purpose you are intending is considered private or commercial use. The metadata of any work licensed under Creative Commons should allow for a contact option.