The constitution actually says:
“Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors…”
If you read the rest of that section and the analysis of it, the underlying facts as to who is an elector has to be based on election day. The legislative would actually be in violation of the constitution if they got together and selected electors against the result of the vote.
There is also something called the Electoral Count Act (ECA), which basically means that you have to select electors according to the law that was in place prior to election day. There is a safe harbor in the law that as long as the electors are designated six days prior (on 12/8/2020) to the electoral college voting (on 12/14/2020) then congress will respect those electors if they follow the rules of the ECA.
Then there is the Supreme Courts decision in Chiafalo, which very clearly describes respecting the vote as a tradition that goes back over 200 years and to go against it would be a major violation of due process. There is simply no way the Supreme Court it it wants to remain legitimate rules otherwise. To do so, would be to end American Democracy.
Basically, there is no scenario under which this somehow falls to the legislatives and Trump ends up as President.
So you suggest this theory as to fraud. Well then you have an issue that the Supreme Court of the USA is not a trier of facts in such a dispute, so the question as to fraud is actually determined by the State courts and not the Supreme court. So the only way this gets to the supreme court is if you first find substantial fraud at the State court and you don’t like the ruling of the State court on the legal implications of that.
So if you think about this, you have a big task ahead of you if you want this to be overturned on fraud allegations as you would need to find substantive fraud before the State sends its electors to congress and be able to make it to state court . Without that you just have a legal question as to what happens if there was substantive fraud, which is irrelevant without the evidence of fraud in the first place.
By comparison with Bush v Gore there were no substantive issues that needed evidence, rather it was pretty much a strict legal question. The facts were not in dispute, rather the legal questions where in dispute. Accordingly going to court was a much faster process.
In any case, Bush v Gore met the ECA requirements and hence no potential issues afterwards, in this case, if fraud were somehow ruled to be so substantial as to call into question the election (mind you, it would likely require new elections for other offices in that state as the whole election would be tainted… not going to happen as too many republicans just won elections) then there is simply no way that the House of Representatives will accept republican electors electing Trump as the ECA requirements will not have been met…. So we get back to either Pelosi is acting President on Jan 20th or the House ends up selecting the President and the Senate the Vice-President. I would not be surprised if Biden agrees to step aside (due to his age as the democrats in congress will not want to risk having the President die and getting a Republican as president then) for the good of the party to allow Kamala to get the Presidency in that case.
TL,DR – You are wrong about what the legislatures can do. Bottom line, if Trump is successful in proving fraud (good luck with that) and undermining the election we either get Pelosi as acting president on Jan 20 or we have Kamala Harris as President with a republican vice-president. There is simply no legal scenario in which Trump wins this election.