Okay, here’s a hypothtical situation that popped into my head today.
A governor is having problems with a person, a spouse, problematic reletive or aide. That person turns up murdered at some point, and their killer caught. Mindless of the political consequences, the governor pardons the killer before there is even a trial. Perhaps before there is even much of an investigation.
In most states, would this hold up in court, would it be like the man had never commitied a crime?
And to extend the question a little, what happens if, 10 years down the road, it’s found that the governor hired the killer in the first place? Does that negate the pardon?
Ultimately this is a question that probably cannot be answered except by waiting for an actual case to develop and then to see what happens.
In my view, pardons are the exclusive province of the Executive, and the only remedy for improvidently granted pardons is to not reelect the governor or, in the most severe case, possibly impeachment. I think I can say without fear of contradiction that this was the generally accepted view. However, some critics of the Clinton Administration threatened to try and reverse his pardon of Marc Rich. While such talk quickly died down, if it had happened I would argue that it was an unconstitutional usurpation of Executive authority and in addition was a bill of attainder. Who knows what the courts would say? But ultimately, you’d think that if a legislature could overturn an Executive pardon then it would have happened either when Ford pardoned Nixon or when Bush pere pardoned Weinberger, et al. in what looked to some (many, even) like an attempt to derail an investigation into his own role in Iran-Contra.
So to answer the OP, I think what would happen is that the pardon would be considered sacrosanct. The fact that the pardon was given as part of a criminal conspiracy has no bearing on its effectiveness – when a governor is elected, it is among other things a statement by the populace that they trust him to wield the pardon as he sees fit. Of course, the governor himself would not be immune from being charged with murder as an accessory.
The above is based on an analysis of generally applicable notions of the balance of Executive and Legislative powers; it is certainly possible that a particular state constitution has limited the powers of its governor, including the pardon power, and such limitation would presumably be effective. But absent such an explicit limitation, the above is my guess as how things would shake out.