There’s no case law, because no president has ever tried.
Were he to try, and were Joe Blow to commit some crime in reliance on this particular application of the Fuhrerprinzip, the courts would have an opportunity to interpret the relevant constitutional provision and identify its limits. I think they might find that such a fundamental subversion of the rule of law was not within the scope of the Constitution.
I don’t think there’s really case law on it, but I think it is uniformly accepted that you cannot. Ex Parte Garland sort of says it (the Court held that “The power of pardon conferred by the Constitution upon the President is unlimited except in cases of impeachment. It extends to every offence known to the law, and may be exercised at any time after its commission, either before legal proceedings are taken or during their pendency, or after conviction and judgment. The power is not subject to legislative control.”) (emphasis added). And, I think that’s still the common understanding of the scope of the pardon power.
Another argument refers to English law and the idea that the presidential pardon power was modeled on the monarch’s pardon power, which did not extend to pardons “by anticipation” (i.e., pardons issued prior to, and in anticipation of, the commission of the offense). Indeed, as I understand it, the forced abdication of James II involved his efforts to do so (or allegedly do so). So it would be unlikely that the US pardon power would extend there.
Almost certainly, yes. A pardon can be issued any time after the commission of the offense. Although you would have to have a pardon drafted that broadly. I think this is more commonly an issue in immunity deals where you need to be careful on how you draft them so that everyone is clear on what is (and is not) included.
Yes. President Ford granted “a full, free, and absolute pardon unto Richard Nixon for all offenses against the United States which he, Richard Nixon, has committed or may have committed or taken part in during the period from January 20, 1969 through August 9, 1974.”
As far as I know, this was never challenged in court. IMHO, any challenge would have been unsuccessful.
Now that we’re talking conspiracy theories, it might be worth pointing out that it would cover potential federal crimes from Nixon’s involvement in a fake Apollo 11 moon landing.
On a more serious note, the scenario that the OP describes offers two difficulties. One is that the crime pardoned has not yet been committed. Another is that it covers crimes that would be committed at a time when the pardoning President is not in office anymore. I think this would be even less acceptable than accepting a pardoning power for future crimes committed during the pardoning President’s tenure; accepting it woul, effectively, entrench the powers of a soon-to-be-ex-President indefinitely.