David. Like the examples I gave, the Constitution, being the instrument of compact, created the Union; just like a contract. In fact, that is essentially what the Constitution is, a contract. Without it, or something in its place, there is no union. Now, as was mentioned by another poster, the union technically existed prior to the Constitution under the Articles of Confederation. However, that union was the construct of the Articles. The AoC compact was rendered null and void ( and with it, the old union) when the new compact (the Constitution) was created , and that compact obligated the states as parties to it. Had there been no Constitution or any other instrument of union to replace the old AoC, and it was abolished, the Union would have ceased to exist and the states would have, theoretically, reverted to independent and sovereign entities themselves. Same exact thing with the Constitution, theoretically. Upon the abolition of the Constitution, the states should theoretically revert to independent states with no attachment to the other states. That is, unless there is something else holding the Union together. But that would require the consent of the states, unless, of course, they were compelled by force or threat of force.
That brings me to your correct point that, essentially, the old Union, a voluntary union, was nullified in the 1860s as several states were compelled to adhere to a compact that was designed to be voluntary. This new Union consisted of the non-seceding states who proceeded to conquer and annex the states that left the old compact. That is the republic within which we reside. I don't think I agree that an amendment would count as a new compact unless the amendment fundamentally changed the nature of the Union. The 13th, 14th and 15th would perhaps count and, perhaps, you could argue the 17th as well. My personal opinion is that we are currently operating as a country created n the 1860s and illegally modified by subsequent Supreme Court Amendments (they call them decisions).
This is my (radical) view, and I may be wrong. It's just my 2 cents.