There is a nonmilitary reason the Confederacy lost the Civil War. Think about it.
Some of us have speculated about that outcome, and what you say is only partly true.
Slavery would have continued, since the planters were prospering, for how long I don't know, but the institution wouldn't have "greatly expanded westward and northward".
There is a dry line running from the vicinity of Minot down through the plains and west of Fort Worth, beyond which 19th-century agriculture would have been unable to raise the only cash crops that made it worth sustaining slavery, which were tobacco, sugar cane, and cotton.
When Stephen A. Douglas offered his Kansas-Nebraska Act and dangled the possibility of extending slavery to the Territories north of Kansas, he was offering an empty bag in return for the very concrete Southern concession of the eastern terminus of the Transcontinental Railroad. Only the bottomlands in a couple of narrow belts up the Missouri were capable of sustaining cotton, sugar, or tobacco plantations. The same thing is true of most of New Mexico and almost all of Arizona. Douglas put California into play, but California freesoilers handled the problem themselves and snuffed out the pro-slavery sentiment there; California entered the Union as a freesoil state.
Recognition of property rights in slaves in Northern States by the courts had more to do with retrieving runaways than with anyone's burning desire to establish a plantation in the Old Northwest: the climate of public opinion would have remained forbidding, so that the idea that somehow the South could have forced Ohioans and Illinoisans to accept cotton plantations in their midst is essentially a roorback. That wasn't going to happen, any more than that William Lloyd Garrison was going to establish, in the open, a public waystation for the Underground Railroad in Kentucky or Tennessee.
No, that's not what you said. You said that had the South won the war, a pro-slavery dictatorship would have forced slavery on everyone coast-to-coast. I can see why you want to back away from that.
BTW, how do you propose that they could have forced people to own slaves?
Hmmm, only a few blacks, slave or free, in several territories that had been open for decades. Expansion West? I don't think so. Expansion in Northern states, with their laws to prevent emmigration of blacks into the states? I don't think so.