To address the question about whether expansion of slavery was mostly an economic question, it would be good to compare the (equivalent in modern $) price of slaves in 1840 or 50 to that in 1860. If slaves were dropping in price due to a surplus, then it lends credence to the claim that the main motive for expansion of slavery into the western territories was partly or largely economic.
Recently, professor Paul Finkleman argued in the Yale Journal of Law and the Humanities that the common perception of slavery as a dying institution before the cotton gin’s invention is misguided. “Slaves were a profitable investment before the cotton gin and an even more profitable investment after its invention,” he wrote in 2013.
https://constitutioncenter.org/blog/the-cotton-gin-a-game-changing-social-and-economic-invention