The legislature provided their own slate of electors (that Pence ignored).
The Secretary of State of PA (since resigned) changed the rules, the state Attorney General (Josh Shapiro) took it to the PA Supreme Court (5 D's, 2 R's, who, of course, ruled 5-2) that the "law" was fine. That is how the "law" was changed.
Now, how could it have easily been rectified by the state legislature?
This is 100% false. The PA legislature did no such thing.