“Slavery was *NOT* going to get abolished in a “preserved” Union.”
You mean like in all the free Northern states?
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. )