This whole discussion sees to negate American history pre-ACW. Before the California Compromise in 1850, there was always a balance of Senators from Free and Slave states. Neither Canada nor Mexico could ever be annexed since that would either add "free" or "slave" territory. In fact, the build up to OTL's ACW started when we did take the northern Mexican territories in the Treaty of Guadalupe-Hidalgo, which made the expansion of free v. slave territories a national issue. Look at Kansas in the late 1850s and tell me that you could create a national consensus for further expansion.