We're talking about the unconstitutional actions of the southern states, so let's leave West Virginia for another discussion shall we?
Article IV, Section III makes it clear that the approval of the other states must be obtained through a majority vote in Congress before a state can be admitted to the Union. And only that majority vote in Congress is needed. Mississippi, Alabama, Ohio, none of them existed until Congress admitted them and, in effect, created them. Once admitted, a state cannot change it's status without congressional approval. It can't split up with out Congressional approval. It can't combine with another state or states (which would have the effect of removing one or more states from the Union) with out Congressional approval. In fact, states cannot change their border a fraction of an inch without Congressional approval. Given that Congressional approval is needed for everything else, it makes much more sense to belive that by implication Congressional approval is needed for states to leave than to believe that Congressional approval is needed for every single change in status except secession.
You're not quoting the text because you're creating it whole cloth.
Likewise you can quote nothing from the Constitution explicitly permitting unilateral secession.
Wrong. Amendment X: "The powers not delegated to the United States by the Constitution, nor prohibited by it to the states, are reserved to the states respectively, or to the people."
You admit the Constitution does not explicitly forbid secession, therefore it is reserved to the States and the people.