[Sherman Logan]

Second, all Unionists (then and now) acknowledged a "right of secession", with the approval of Congress

I'm not sure where you get the idea that Congress has the constitutional authority to allow a state to leave the Union. It has the power to admit states, and (with agreement by the state's legislature) to split up a state.

But I don't know of any power to allow a state to leave, unless you extrapolate that from the power to admit new states, which I guess is not completely illogical.

[central_va]

The USC is silent on the issue of secession, neither for or against.

[BroJoeK]

Sherman Logan: "I'm not sure where you get the idea that Congress has the constitutional authority to allow a state to leave the Union.
It has the power to admit states, and (with agreement by the state's legislature) to split up a state."

As in all matters Constitutional, you have to start with James Madison, and that's whose words I try hard to summarize accurately.
Madison's key words were:

"The compact can only be dissolved by the consent of the other parties, or by usurpations or abuses of power justly having that effect.
It will hardly be contended that there is anything in the terms or nature of the compact, authorizing a party to dissolve it at pleasure."

I take Madison's "consent of the other parties" to mean Congress.
And not just me, that was also Lincoln's opinion, and not just Lincoln, but all five of the US Presidents still living in 1860!