“Who decides that? What if there are multiple, competing Certificates? What if the Certificates are obviously fraudulent? “
Congress is the decider in all of the above.
“There are lots of possibilities, and the Framers wanted a Constitutional officer with power and authority to oversee this matter.”
Again, it is congress that is the constitutional authority.
And they failed.
After that, there is no recourse other than impeachment. And we already know who the authority is there.
Pence could have . . . and he “did the right thing” because he didn’t.