Even so, unless those "free" northern states seceded from the Union, (Like Massachusetts and Connecticut tried to do in 1814 at the Hartford convention) the Union would have remained a slave Union indefinitely.
Even worse, all of those "free" state reps and senators voted to enshrine slavery in the US Constitution by passing the Corwin amendment.
(And I don't want to hear any of your excuses saying that was justified. It wasn't. )
Wow, 18 slaves in New Jersey in 1860! You consider that “plenty.”