I'm not sure what you mean. If just a few states with Republican legislators withhold their electoral submission and lowers Biden under 270, does it not go to the House with each State having one vote? It's confusing, but that's how I understand it.
here is a link to the electoral college. I believe it will answer your questions.
https://www.archives.gov/electoral-college/2020
The electors have all been selected. That was done by the voters through the election and the certification by the appropriate authorities in accordance with existing law. The legislatures can't withhold those votes. All they could do is try to change the law for selecting electors after the fact and submit a second slate to vote in the Electoral College. The problems with that, of course, is that the Electoral College votes tomorrow, none of the legislatures in any of the states are currently in session to vote on the change, absent a court order invalidating the election it's hard to believe if would survive the legal challenge, and absent any court ruling invalidating an election no legislature would have done it to begin with even if they were in session.
No.
Say Pennsylvania withholds its 20 Electors.
The number appointed is no longer 535, it's 515, and the number to win is now 258.
Every state that withholds its votes lowers the number of Electors appointed, and therefor lowers the number needed to win.