You are being ridiculous.
Saying fraud without evidence of fraud will make any action by the legislature illegal. The Supreme Court, even with 3 Trump justices, will quickly rule that such an action is illegal. So, no, there cannot be civil war with claims of fraud that have no sufficient evidence as you will have Biden as President in such a case and he will invoke the insurrection act to put down anyone threatening violence against the US. You will not have States getting into such a fight rather some people who are nuts.
On the other hand, if there is some absurd series of events that has precedent and laws trampled all over, you will have the House and the Senate declaring differing people as President. The end result will risk having States pick sides and risking civil war. As to the insurrection act, you will have 2 different presidents who will invoke it, setting up a split in the US military as well.
All I can say, is that this talk of having legislatures decide is dangerous and people should stop talking about it and giving strength to it. The bloodthirsty nature of those who are so righteous that they would push this country to civil war is just disgusting and abominable.