"To presume that since the right to unilateral secession exists because it is not explicitly forbidden, then you must also assume that the Federal government had a unilateral right to expel any state, even against its wishes, since that is also not explicitly forbidden."
Nothing in the Constitution gives the Federal Government the power to prevent states from Seceding.
Nothing in the Constitution gives the Federal Government the power to trade ANY state to any country.
The power of the States is legally defined (as is the power of the Federal Government) in the Document called the Constitution.
You need to study it.
Since you claim that the United States was not created as a "prepetual union" under the Constitution, what exactly prevents a 2/3 majority of the Congress and a 3/4 majority of the other states from selling any given state to a foreign power even if that state does not wish to be sold?