As a follow-up on:
The parlamentary committee of enquiry did invite Snowden in 2013, but the government blocked it - they weaseled around it, based on “cannot guarantee Snowden’s security” and “the US would be mad at us”.
The committee on the NSA affair sued the government, and the Bundesgerichtshof (highest ordinary court in Germany) decided that the government has to invite Snowden as a witness and ensure his security - the rights of the parliament are more important than the national interests as seen by the executive.
…interesting times, I have no idea how the government will react…