I did not know that it was in the air to beat the soon-to-be-born CSA to the punch by annexing the “ignorant and degraded” people of Mexico. Will any more be heard of this idea?
It's the same way nowadays. Every whisper makes it onto the Internet, and the whispers the major media like make it into their articles and broadcasts. Those they don't approve of circulate underground. Then there's the rumor about Buchanan's administration arming the rebels, which turned out to be true so far as some key officials were concerned.
Also familiar from our own day: Nobody knows what's really going on. Was secession for real? Or just a trick or bluff or stunt designed to get concessions? We now know the secessionists meant it. That wasn't obvious at the time. What the country would do in the face of secession attempts also wasn't entirely certain.
Then as now, politicians make all kinds of last-minute schemes to deal with what's going on, schemes that usually fall through. What is different now is that we have think tanks and all manner of organizations and groups whose full-time job is to think, plan and scheme. Some small-town lawyer in downstate Illinois didn't have those resources, so the idea that he came up with and implemented some far-seeing, long-range master-plan is more than a little bit silly.
Dealing with the immediate situation was challenging enough, and nobody knew (or deep down wanted to know if what was coming was depressing) what tomorrow would bring, let alone how things would look over a century later.