Who decides that? What if there are multiple, competing Certificates? What if the Certificates are obviously fraudulent? There are lots of possibilities, and the Framers wanted a Constitutional officer with power and authority to oversee this matter.
You didn't suggest any reason why the Framers assigned the task of opening Certificates to the President of the Senate. The fact that he presides over the Senate does not mean that he has to be the one to open the Certificates.
Consider that this is the Constitution. It is not a statute book or a code of regulations. In a Constitution only matters of large importance are dealt with. Details are left to statutes and court rulings, etc. Given this, why did the Framers think that opening Certificates was so important that they had to write in the Constitution that only the President of the Senate is allowed to do it? The answer is that, mindful of the gross frauds and skullduggery that occurred under British colonial government, they wanted a check, a safeguard that things were on the up and up. It's one of the many "checks and balances" in the Constitution. With Pence's misfeasance, we did not get this.
As for writing that excludes this power, the 10th Amendment covers it well. Any power not specifically granted is non-existent.
The Tenth Amendment is irrelevant to this question. The Tenth Amendment governs the division of power between the Federal Government on the one hand, and the States and the People on the other. It says nothing about the power of the President of the Senate within the Senate.
However, if you do have a cite to, again, some passage from the Debates on the Constitution, or the Federalist Papers, or the Anti-Federalist Papers, for the proposition that the opening of the Certificates is a mere ministerial task, I would be interested to learn of it.
“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.