Once again, your lack of reading comprehension is only exceeded by your manifest ignorance of actual history.
First, there is no secession mechanism specified in the Constitution, doubtless because our Founders hoped there would be no need for it.
They did, however, provide numerous mechanisms for changing laws, challenging legal interpretations, and changing or rewriting the Constitution itself -- so that's what they hoped for the future.
Founders' views on "disunion" were all consistent with the ideas that it should be by mutual consent or from some material breach of contract, such as "oppression" or "usurpations".
So those qualify as "original intent".
Second, no effort -- zero, zip, nada -- no effort was made by two US Presidents (Buchanan & Lincoln) to, in your words, "force states to remain in the Union:.
What Lincoln did, instead, was first: unconditionally defeat the alien military power which first provoked, then started and formally declared war on the United States, then second, Presidents Lincoln & Johnson allowed their citizens (all of their citizens) to elect representatives to Congress and to presidential electoral colleges.
Third, again: Founders' original intent was that lawful secession come through mutual consent (i.e., approval of Congress) or by some material breech of contract.
Therefore, US "secession" from the old Articles of Confederation was 100% lawful, while Deep South slave-power unilateral declarations of secession were 100% illegal, period.
No secession mechanism...again you are assuming that if it isn’t specified as allowed, it isn’t permitted. They knew how to write and include the words “Perpetual Union”, and they chose not to. Even with “Perpetual Union” in the Articles, states seceded from that pact. You don’t read carefully, you don’t know the relevant literature, and you have trouble reasoning. In particular, the doctrine of enumerated powers and the Ninth and Tenth Amendments are a mystery to you. The South, New England, New York City, and others at various times believed they had the right to secede.
You are simply a big government conservative who doesn’t want to recognize what historians already acknowledge; namely, that Lincoln was a dictator who acted largely outside the law. The American Constitutional Republic today is as dead as the Roman Republic in the days of Augustus Caesar, and it is largely the fault of generations of faux “conservatives” like you.