Really Mueller has done about every possible thing he could to limit things to an extremely narrow scope, remain objective, and not overstep his boundaries -- all in effort to avoid any kind of political activity and partisanship. In fact, it seems this has been most unfair to Democrats as his fear of Republicans trying to invalidate the work for partisanship drove his choices rather than following the facts of the case.
And STILL he's in this mess knowing 1. his job is incomplete as perception of his work has been marred and 2. he can't set the record straight without inviting bullshit claims of partisanship.
At this point I think he should abandon that worry and just be as direct as possible about his findings.