But I note that you don't address why the Framers mysteriously chose to remain silent on this issue when they had spoken loud and clear in the Articles. Assuming, arguendo, that "it's not a stretch to conclude" that Congressional approval is needed for secession, what possible reason would the Framers have for not explicitly stating that states did not have the power to secede? After all, as you stated, the Framers made a laundry list of powers vis-a-vis state admission, division, and borders. Why would they go silent on secession?
I don't know, but nothing says that those were the sole powers. If Marshall is correct and interpretations must be made based on a broad reading of the entire Constitution then it makes more sense to think the authors of the Constitution would explicitly state the exceptions rather than leave them to be assumed.