... the declaration was of independence.  The states were not then united into one nation, though they were united in opposition to Britain and in support of independence.

... in 1776.  However, it is now somewhat common for people to infer that one is referring to the constitutional republic of the "United States of America" when one writes the phrase "United States of America."  

As to "that discussion" you made some very good points.  However, I believe you might improve on your argument by recognizing that a threat (whether real or perceived -- and I would say real, but exaggerated) to the perpetuation of southern slavery was the catalyst for secession, and that secession was the catalyst for war.

Further, secession was not a popular southern cause, prevailing in Georgia, for example, only because a popular vote was studiously avoided by the patriarchs in charge, and because, according to no less of an authority than Alexander Hamilton Stephens, the election for delegates to a hurriedly called and gerrymandered secession convention occurred on a rainy day favoring pro-secession town voters who didn't need to slog quite so far through the mud.

So count me among the number of those who don't cry for the Confederate loss and the losers who  instigated that ill conceived, unfortunate, and anti-democratic misadventure.  They got what they deserved.  Unfortunately, so too did many innocents, forced into an unnecessary war chosen by a cadre of perfumed antebellum chicken littles.  And the only good that came of it was the accelerated demise of chattel slavery.  At least we have that to celebrate.