The President of the Senate shall, in the presence of the Senate and House of Representatives, open all the certificates and the votes shall then be counted ...
Guess what -- that really IS it.
The only discretion Congress has is in cases where one state has multiple certified electoral votes. That was not the case in 2020.
No; there is no limitation in that text. Do you see the word “only” anywhere?
Not to mention “the votes shall be counted” does indeed imply legitimate votes.