The action had to have been committed is the only requirement for a pardon. You can't preemptively pardon but you can pardon any act that has already happened even if Justice has not been involved yet.
There is SCOTUS precedent for this.
First, it's common sense in the way the Founders wanted to have checks and balances. The Judiicial branch can prosecute and render judgment or harm. The Executive can bypass the Judicial with Pardon power. And the Legislative branch bypasses the Judicial via Impeachment. Each branch of government has a relief mechanism
Second, I had to look up the case, damn you for that. It's Garland 1867. There's been some twiddling back and forth since then. But as a constitutional matter I suspect a conservative SCOTUS would say that the Pardon power is pretty absolute.
The issue also came up when Trump trolled liberals by saying he could pardon himself. The president is immune from anything that goes through the Judicial branch, with some exceptions. An Impeachment originates from the Legislative where it is explicitly stated and understood that the president cannot pardon himself from Impeachment.
Regarding secret pardons, I don't care. If it's already a part of the judicial record then the pardon could not be secret. But if there is no judicial record I suspect a pardon could become a matter of national security and not be known by the public.