1) FBI has already stated that "Russians" put out ads for Bernie, Killary and Trump, now it's only one candidate? 
2) Which DOJ policy is that?
3) If he had evidence that Trump committed a crime he would have charged him or presented it to congress, or is Mueller hiding that evidence?
4) Congress to act on what? With no evidence present, what would you like congress to accuse Trump of doing?
Let me help you out.
1, Russia wanted to create turmoil and dissent among americans. Originally it was wide spread with many candidates. When it came down to Hillary and Trump it was clear Russia sided with Trump.
2. The policy blocking indicting a sitting president dates back to the presidency of Richard Nixon. In September 1973, just under a year before Nixon resigned, the DOJ’s Office of Legal Counsel determined that a criminal case against the president “would interfere with the President’s unique official duties, most of which cannot be performed by anyone else.” Therefore, impeachment is the only manner by which a sitting president can be penalized for wrongdoing.
3. See # 2 and read the Mueller report where he goes into detail about it at the beginning of Volume 2
4. He outlined 10 instances, 4 with "substantial evidence of obstruction" that he felt was up to Congress to pursue, since {see #2)
Now I personally think Mueller was a coward for not being more decisive either way. There was nothing legally keeping him from saying "But for the fact we could not indict him, under other circumstances he would be indicted." Instead he went with a limp wristed "If we were confident he did not commit a crime we would have said so"
