1) Congress was never granted authority to “certify” anything. This is a constantly repeated FR legend. Congress opens and counts the votes in a public session to avoid deception.
2) State Legislatures appoint their Electors. This is a plenary power, there is nothing over it. A State Legislature could have appealed to Congress not to count the ballots sent under seal until the State Legislature completed an investigation, but none did so. None even convened to discuss it. The Pennsylvania Legislature fled to avoid the issue.
Just read the entire darn report.