So rogue elements of a state government could send any list they chose, and the VP is bound to open it.
No. Each state, via its public laws, has a process for certifying its electors. That process isn't hidden, so the identify of the electors and how they voted isn't secret. Assuming the transmittal envelope has the correct certification on the outside, that's the only one that would have to be opened.
But lets assume for a second that someone slipped in a second certification envelope, and you couldn't tell from the outside which is legitimate. What would almost certainly happen is that both would be opened, and the information inside would be compared against the publicly available information regarding the identify of the official electors and how they voted.
The Constitution, by the way, doesn't state who actually gives the "official count" of the ballots. It doesn't say Congress, and it doesn't say the VP. And I think that's because that action was viewed as ministerial -- simple math and acknowledgement of the totals rather than anything to which some discretion was attached. They were just witnesses acknowledging an undisputed fact, not someone making a substantive decision.
