While I agree on nearly all points, I've not seen anything in print to suggest that Southerners did not respect Walker or regard his demise as the result of betrayal. While you were away, I reproduced a good artilcle on Walker which demonstrated the support he enjoyed among influencial Southerners. Had his effort been thought of as a farce of some kind, it seems unlikely that he would have been honored in print after his death.
With regard to your other points, here's an article on the impact of slavery and the admission of California as a free state in 1850. California's admission in 1850 tipped the balance by one of free states versus slave states. Note that I am agreeing with your points. I disagree only that 1) Southerners were willing to concede that their lot in the Union would decrease, while the free states would increase, and 2) that Southern agitation about "states rights" in the territories had much to do with the suitability of western lands for plantation agriculture. A marked decline in political power was not something most Southerners would accept, despite the obvious facts, nor did they particularly care how many slaves actually tilled the soil in new slave states, should any be admitted.