The Texas situation, I'll admit that wasn't viable for Mexico, and it was just plain stupid of it to try and hold on and perhaps, a case could be made for the unrealistic policy adopted by Mexico in holding its other states on the US border too.
The fact was though, once the war started neither side seemed willing to treat, as that source I had said. Mexico because they were fighting to the bitter end and America because, I assume, they had plans to ensure the expected seizure of territory went off without a hitch. The US were really the only ones capable of defusing the situation before the war started, at least in my opinion. Texas was a hot topic, but let's think realistically here; do we really believe that just because Mexico claims hegemony over Texas that Texas, or the US for that matter will take that claim seriously when Mexico has no means with which to back that claim up? Additionally, say you were the US and you didn't want a war, would you allow the fact that Mexico claimed part of your land act as a cassus beli?
This is really just me rambling at this stage, but the fact that Mexico was so clearly vulnerable to attack is no secret; just look at the damage the Native American tribes were able to inflict without barely a response from Mexico.
Conversely, if you wanted to look at it from the point of view of the US you could say that they were trying to secure their borders and ensure internal security. But Manifest Destiny comes into it again, and was actually used by many in the US to justify their actions with respect to the war once it was over and Mexico had been left a smoldering shell of its former self. Could the US moves be justified simply because it was in their imperial plans to expand in that direction? I don't think it could. Although I would also argue that it was only a matter of time before the states joined America anyway.
Texas had rebelled without direct US help and had achieved de facto independence from Mexico, California would rebel itself in June 1846 just as the war was about to begin; this was before they knew US soldiers would soon be fighting with them by the way. New Mexico was perhaps more secure, but the fact that two of the three states supposedly under Mexican control broke away without America speaks volumes about their future had the US not invaded.
I'll let you draw your own conclusions here, but I hope you'll share some of you opinions on the subject, and remember that WDF 11: The Mexican American War is out now! Thanks for reading history friends.