It's not just that Mueller was part of the German language group at the Synod, it's also that the Synod was responsible for writing it's own report, not the CDF.  To say Mueller had no input in AL is incorrect.  He had every opportunity to offer input and did, as you say, vote with the rest of the German group in the affirmative on the solution to the divorce question.  If Pope Francis really wanted to sideline Mueller he would have done so in 2013 and not elevated him to Cardinal.