> hypothetical example of a good preemptive pardon would be Congress repealing an unjust law, and the president pardoning anybody who broke that law before the repeal

Congress could do this when they pass the law. If they didn't, they specifically chose not to.

> less convinced about preventing family pardons. Those people (generally) aren't politicians

What if we invert the question: in what case would the family require a pardon such that their spouse or parent in a position of massive power couldn't help them out of a legitimate scuffle?

> Most people don't follow politics closely enough to know who's been pardoned

Then why do most of the controversial pardons come in this envelope?

> how about allowing the House or Senate to veto a pardon with a 2/3 majority?

I like this much better.