Although the DOJ has a policy against indicting sitting presidents, Mueller had options to secure indictments for Trump upon leaving office. He did not pursue those options. It was not “impossible” to charge Trump. Moreover, Mueller was free to opine that Trump committed a crime without inducting him. He declined to do so.
Second, Congress does not have prosecutorial authority. That function is reserved for the Executive branch, which in this case determined that no crime was committed. Keep in mind that the Special Counsel does not report to Congress; he is a DOJ employee, and all of his decisions are subject to review by the Attorney General. The AG makes the final call. Up until 1999, there was an Independent Counsel statute that gave Congress the ability to initiate special counsel investigations and empowered the prosecutor to make recommendations on the issue of impeachment. Congress let it expire. Long story short, Congress is stuck with Barr’s conclusion on the obstruction question.