There is a state-by-state countdown of the last date an independent may file to appear on the ballot fr president.
For some key states like Texas, Illinois, Indiana, and North Carolina, that date is already past. Many more states will close to independents in the next 6-8 weeks. A campaign starting now couldn’t get enough votes to even be a spoiler.
The chusing of electors is a plenary power of State Legislatures. There are no rules and no laws that a State Legislature cannot change prior to November 8, and possibly prior to December 12.