As I understand it, he's trying to prove motive, which means he's trying to make sense of Trump's brain.
Good luck with that. We don't try to understand sociopaths because they think abnormally. We just observe their actions (eg killing people) and accumulate whatever is available to confirm their intentions. With Trump, we have so much evidence in every direction that he does these thinks wilfully. Incites riots. Yep. Treats national secrets as his own property. Yep. But at some point, you have to indict and rely on prosecutors to make a persuasive case. I think we long since reached that point. And yet here we are two years later and he has not been indicted.
Garland will be on the merry-go-round of Trump's brain forever unless he finally just commits. Time matters. Trump needs to be removed from participating in this election asap.