> If the President was still in office and didn't object when you tried to invoke the advance-written pardon, it wouldn't seem as though anyone else would really have a place to interfere.
If the President is still in office, as head of the executive branch he can prevent prosecution even if there is no pardon, and, anyway, he could just issue a new and properly retrospective pardon. A pardon that's only good if the President that issued it is still in office and still agrees with it is exactly the same as no pardon at all.
The pardon power is only meaningful because it is binding on the federal government, and not just as long as the current President is in power, but forever (or, at least, until the Constitution is amended to change its status.)
AFAICT, the general consensus among those who study Constitutional law is that prospective pardons (those for crimes not yet committed when the pardon is issued) are nullities; for there to be a crime for a pardon or reprieve to apply to, it must have been committed (even if it has not been discovered) when the pardon is issued.
But because no prospective pardons have been issued and then their effect challenged in court, this has never been litigated, and there is no case law on the subject. (But if prospective pardons were possible, given the established breadth of the pardon power in other aspects, it would mean that the President could essentially prevent the US government from ever adopting -- or, rather, ever enforcing -- certain criminal laws that do not yet exist, simply by issuing prospective pardons for any offenses resulting from particular classes of conduct that he doesn't want to ever be criminalized.)