Missouri was more pro-Union than most CS states too (OOC: think Tenn.), so perhaps that could have been avoided? IMO though preventing Maryland from seceding would be a good idea </understatement>. But I think this is doable. Although it is a shame, New Washington would never get built out West... 'course us Unionians usually just call it plain ol' Washington since the CS renamed our old capital Spartacus after their Revolution.IC: Well, Kentucky was pretty close to staying in the Union. We all know about Kentucky's importance in allowing the Confederacy to attack vital industrial centers in Ohio.
Actually now that I think about pretty much the only good that came out of the Mexican War was California. The US could have possibly held together otherwise, avoiding the Confederian War of Independence and the Mormon War of Independence. Although after Polk got all of Oregon he had to get land for slavery to balance it out. Which is why the Wilmot Proviso ruined everything. It might have worked if we had just annexed Upper California and Texas and New Mexico to connect, but annexing California (including Baja; yes everyone forgets it was actually a different territory instead of always united with the north), New Mexico (including what's now Arizona), Rio Grande, Sonora (OOC:Including OTL Sinaloa), Chihuaha (OOC: Including OTL Durango) was unneeded. The latter states in particular. Texians are horrible overlords to the Mexican minorities in those states. It's easy to sympathize with the insurgents, although veterans of the Rio Grande War probably see it in a different light.
I just thought of Calhoun. Maybe he could die earlier instead of shortly after his term as CS President ended in '55. He was quite old. Not that an able President mattered that in the states'-rights paradise of the CSA, but Calhoun radicalized the South a lot when the Wilmot Proviso passed.