In a normal situation, yes.
In the case of secession, who is the arbiter? A court controlled by one of the intersted parties.
"No man is allowed to be a judge in his own cause, because his interest would certainly bias his judgment, and, not improbably, corrupt his integrity."
IMHO, this quote applies to a government as well.
You have stated several times that an ammendment would allow an individual state to leave the union - I assume you mean that a state could request to leave and 2/3's congress and 3/4 states must approve. If so, why? If the power of the state is derived from the people (and the power of the federal state derived from the people and the states), the people have the right to determine whether the nation government is serving thier interests and, failing other remedies (see the earlier link to Madisions Essay) may remove themselves from the union.
Madison wrote: "The influence of factious leaders may kindle a flame within their particular States, but will be unable to spread a general conflagration through the other States. A religious sect may degenerate into a political faction in a part of the Confederacy; but the variety of sects dispersed over the entire face of it must secure the national councils against any danger from that source. A rage for paper money, for an abolition of debts, for an equal division of property, or for any other improper or wicked project, will be less apt to pervade the whole body of the Union than a particular member of it; in the same proportion as such a malady is more likely to taint a particular county or district, than an entire State."
Unmention is what happens if such events DO occur - what is the remedy?