let's do the seal team six hypo. president orders st6 to assassinate a political rival bc rival is a threat to national security. possible this is absolutely immune under pres's commander in chief power. if it's not, then it gets presumptive immunity (which may be absolute anyway)
Presumably you'd need to collect evidence that the presumption shouldn't be granted, and that evidence would probably be in the conversations with officials which themselves are at least presumptively immune and therefore need their own presumption overcome before they can be used as evidence.
There's also the possibility that Der Orangenführer just spouts his illicit motivations at a rally in public, but there's not a lot to stop SCOTUS from closing the "public-statements loophole" next session.