> A reasonable opposition party would declare the pardons invalid. Is that a valid interpretation of pardon power, does that undermine the legitimacy of our laws?
No, in the USA the pardon power belongs to the President. Only a constitutional amendment could invalidate pardons.
As we are finding out in real time, the President has the power to try to do a million things, legal or illegal, constitutional or unconstitutional, and then whichever ones don't get pushback defacto become actual powers. Throw something at the wall, if it sticks, then it's a Presidential Power. If it doesn't, there's no consequences. Just shrug and throw something else at the wall.
There would still be a valuable public record produced by the investigation and court proceedings. Going after pardoned criminals is absolutely something the next administration should do. (We have zero precedent for preëmptive and blanket pardons in our courts, for example.)
Only a constitutional amendment could allow the executive to declare war, or regulate trade, or control funds, or countless other unenumerated or explicitly disallowed powers. But if those rules are broken, we should still follow the rule that says the executive is immune from punishment and can declare anyone else forgiven? If that's the case there's actually only one rule: the executive is king. You can wipe your ass with the rest of the constitution.