He already knows what happened. He is trying to make it admissible in court. With Flynn turncoating on Trump, we are likely closer.
The video likely is real. The Russians own the Trumps.
When the time is right, Mueller will leak he is going to indict Donnie Jr and then by his idiotic instinct Donnie Jr will leak all of his incriminating emails on Twitter. Mueller will then have Jr in his hands.
I think it may be easier yet than that... Trump did not expect to win the election. obviously, no one did. He expected to be on the sidelines where he could hurl unsubstantiated claims as a private citizen and claim its all rigged against him... bla bla bla...
He needs an out. He wants an out so he can get back to playing golf and laundering money through his real estate dealings, like he had been doing in NY for decades with the Russians. His out will be threats of legal action against Jr. He will be able to resign under the guise of the system being rigged against him like he originally planned, and that he is 'reluctantly' stepping down to protect his children from the evil dark state, or whatever.