If I’m not mistaken, the legislatures of PA, MI, WI, GA and NC are all GOP and can decide which slate of electors to appoint if it is clear there has been fraud.
Yes, the state legislatures can decide which Electors to submit. However, the Governor of the states normally have to certify the Electors and it’s the Governor who submits the Electors (to VP Pence as President of the Senate, the state’s Sec. of State, and a Federal judge).
In war room gaming the Dems did a few months ago, Democratic Governors would refuse to submit Republican Electors for their state if the election was close.