I recognize that this isn't your argument, but it isn't an argument at all. It is a statement that lacks in support or analysis and cuts against what are (at least now) standard tools for textual analysis. It is--at the very least--a stretch to think that the phrase "more perfect" means perpetual, but it is even a greater stretch considering that most of the exact same folks who wrote the Articles (and therefore were perfectly aware how to state that a union was perpetual) would suddenly, a little more than a decade later, take to what amounts to a coded message to convey the same idea.
I know I'm restating my argument, but the Framers wrote that the Union was perpetual once: if they wanted to do so again, they could have. But they didn't.
I cannot believe that given the other restrictions the Constitution places on the states and the powers granted to Congress to literally create states in the first place that the Founders meant for secession to be unilateral.
Why not? Again, the fact that all the other restrictions are spelled out so carefully tends to cut in favor of thinking that the Framers intended for the states to have the right of secession. When the Framers were drafting Art. I, Sec. 10--which are state prohibitions--do you think they just forgot to include secession? When they were drafting Art. IV, do you think the subject slipped their mind again? That would seem like a strange result, especially considering that the United States just fought a war of secession against England, don't you think?
Again, the Framers were intelligent people; we have to assume that any omissions were deliberate. If we don't do this, we needn't bother with constitutional interpretation at all: it becomes meaningless because anything omitted is just assumed.
Or is it as Chief Justice Marshall pointed out, that the authors of the Constitution gave the broad outlines and meant for much of the meaning to be interpreted based on those outlines. So if the Constitution gives Congress the power to admit states in the first place and to approve any change in their status or border once they're in then I don't think it's any real stretch to conclude that Congressional approval is needed for them to leave. If states cannot take steps which impact the interests and well-being of other states without consent of Congress while they are a part of the U.S. then why should we assume they can take those steps while leaving? To my way of thinking assuming that the founders meant for the Constitution to be used as a club to beat up remaining states if a particular state wanted to leave is too far fetched to believe.