Alan,After reading through the thread, I am still at a loss to understand the end goal.
(a) What do you believe that the SC Sec Dec was supposed to say? If you were writing their Secessions Declaration, what do believe it should have included?
(b) What is it that you feel they were supposed to say that they didn't say?
I would note that the constraints of time, precision, brevity, and other things, limit what they could or would say in their Declaration. They were not writing a laundry list of issues, they were trying to get to the point, in as persuasive a way as possible. They were writing what they knew was a momentous document, and they were writing a document that would stand the test of time. A persuasive argument for secession does not necessarily have to include everything there is to say about secession. Things that might be "true" but not persuasive would and could and maybe should be omitted.
I believe they wrote the truth they thought was important to tell, which distilled the essence of their argument for dissolving the Union. Of course they could have written more. But the things that were essential for them to say, they said.
As an aside: we have all written stuff that, on review, we could have improved upon. The SC Sec Dec might have imperfections that are due to people being imperfect, as opposed to, not being truthful.
EDIT: It might be that per your reading, the Sec Dec is not persuasive. But they were writing for their peers, and if it worked for their peers, then they accomplished their goals.
I just launched the new thread prompted by your thoughtful post. I look forward to your answer(s) to the question.