You totally could arrest them if you had proof of a crime, which in the case of those two is certain

There are two ideas that Q followers seem to confuse for the sake of hopium, one being the need for precedent to arrest former elected officials, and the other is needing to already be in court to "introduce evidence"

Both are wrong. Trump can "introduce evidence" until the cows come home during his trials but the evidence already exists and no court will hear it. "Setting precedent" makes no sense when there's no need to set it