The case against Trump only needs to be made in the heads of the American people, as opposed to in the courts. The legal perspective of Trump being charged and convicted will be a bonus for the FBI/DOJ.
Trumps people are nationalists or ultra-nationalists and any hint of treason on the part of their leader is going to be a bitter pill for them to swallow.
Witness how C.A. and others are stumbling around with attempted insults and spamming as opposed to any talking points that could work in Trump's favour.
FBI/DOJ mission:
1. influence the coming election.
2. Diminish Trump's support so that he will not run for president.
3. Save America from a fascist takeover of its democracy.