Western provinces would not have automatically assumed they would join the Union -- but on the other hand, if the CSA was combustible, or relatively poor, or did not believe in unity, why join it? Would Texas have stayed in the CSA once oil was discovered? Why? I think it more likely that Texas and Louisiana would have held their noses and allied with each other, because of oil and the Mississippi River. They would be better off doing business on their own with the CSA than joining it. The north central territories would NEVER have joined with the CSA, port or no port, but would do business through a foreign New Orleans.
Once you no longer have northern sheriffs being forced to observe the Fugitive Slave Act, the CSA's northern boundary would always leak. Imagine a Berlin wall stretching from the Atlantic to the Mississippi! Would the CSA have the leadership or the money to make it happen? If you were a southern soldier with a small-holding and a slave or two, or none, once the dust had settled and your so-called honor was no longer threatened by the Yanks, would you feel like paying taxes to Richmond or sending your son to protect that boundary? It didn't work for the British between 1763 and 1776, it didn't work for eastern businessmen in 1792, and it didn't work for East Germany in 1989. In fact, it didn't really work for East Germany between 1961 and 1989 either!
I haven't read this anywhere and I'm not spouting another author's ideas -- I've just been tossing this around in my head in the few minutes since reading Gail Collins's blog. If I can come up with such serious reasons for the CSA not to stay together in just a few minutes, I don't think it really could have survived.
The north has been subsidizing the south since the writing of the Constitution: the three-fifths compromise and the siting of the capital on the Potomac. The south has been holding the north for ransom -- almost literally, since we've been putting our military bases there -- ever since. After the late 1860s unpleasantness, it received northern industry because carpetbaggers saw a fresh wilderness to exploit. Before globalization, it received northern industry because southerners were willing to sacrifice their dignity, working without unions. The south would have been better off listening to Robert E. Lee's better angels than sending him off to fight.
(Incidentally, Lee himself did not always listen to his better angels. You can read here about how he treated his own and inherited slaves.)