" If they'd been allowed to fully count their slaves as if they were eligible to vote, slavery would have become the law of the entire land rather than an increasingly marginalized system that was facing extinction. It's why the South seceded. They saw the handwriting on the wall."

If you start with the premise that the South was entitled to representation for all of the people that they gave no rights whatsoever, then your theory makes sense. The Electoral College and Three-Fifths clause were about appeasing the South into believing the North would never have the power to eliminate slavery. They might never have had Southern states seceded. The Emancipation Proclamation wasn't about ending slavery but disrupting the Southern economy and keeping Britain and France from siding with the Confederacy.

Where the South went wrong was not paying attention to that clause in the Constitution (Article 1, Section 9, Clause 1) that guaranted them twenty years before the country could even think about ending the Transatlantic Slave Trade. Little did they know that Jefferson et.al. were planning to let states like Virginia, Maryland, and Delaware profit from their excess slaves after they jacked up their fields growing tobacco without rotating crops. That cut out the leading ports like Charleston from importing slaves from Africa and forcing them to pay higher prices. It also meant to meet demand they incorporated forced breeding and rape of enslaved women. Now That's the sort of history America never taught because it might make people feel bad.