Trist put himself at serious risk OTL with his actions in defiance of President Polk so I seriously doubt he'll dare to yield territories already in the hands of the US, let alone parts of Texas for a decade already.
Grimm,
Agreed. The treaty Trist negotiated and why Polk accepted it has more to do with domestic US politics than anything else. While the war was ongoing, it was popular the US population. Many US congressmen and senators had a different view however.
While the war was still in progress, the House passed a resolution thanking General Taylor for his service. Added to that resolution was an amendment chiding President Polk for "unconstitutionally" starting an "unnecessary" war. The House even began investigations into how the war began, foreshadowing the powerful House committee that examined the conduct of the Civil War during the war.
Towards the war's end even more opposition arose when it became clear that the US was going to acquire a huge territorial cession from Mexico. Whig opposition was especially fierce, it included Congressman A. Lincoln, and Whig newspapers slowly began effecting public opinion, especially in the North and Midwest. The war quickly became viewed as a little more than a Southern landgrab. Grant in his autobiography famously called the war "wicked" and Trist himself, although pro-slavery to the point of forging slave sales documents during his tenure as US consul in Havana, said the treaty made him ashamed as an American.
That gives us an inkling of the domestic political scene Polk faced when the Trist treaty arrived in Washington. He's been denounced in the House, his machinations that started the war are being investigated, and sections of the country are upset over the size of the cession. Polk also had other troubles, this time from the war's domestic supporters. He needed a bone to throw their way too.
In a weird way, Trist gave Polk precisely the treaty the president needed. Without actually knowing it, Trist negotiated a cession that was just big enough to satisfy the war's supporters while also being just little enough to assuage the war's detractors. The fact that the ceded lands contained relatively few Mexicans probably helped with the latter too, the Senate actually amended the treaty in order to remove those sections granting Mexicans US citizenship within a year.
Domestic
Mexican politics is also part of the equation. Trist and Scott loathed each other at first, but soon became great friends. Scott surely impressed on Trist the precarious nature of the US occupation of Mexican cities.
Between seizing Mexico City in Spetember of 1847 and the treaty being signed in February of 1848, the US army didn't stop it's actions and the Mexicans didn't stop theirs. The US launched expeditions against cities south of Veracruz and the Mexicans continually harassed US supply lines between Veracruz and Mexico City. This situation couldn't last for long. The Mexican government negotiating with Trist would lose what little control they had over the country or the US would find it necessary to begin major operations again. Scott was also facing that most American problem with his army; now that war was "won" everybody wanted to go home whether peace was actually at hand or not.
Trist must have had these Mexican domestic and US military issues in mind, especially given his friendship with Scott. Accordingly, Trist got as much as he could out of Mexico as quickly as he could and without also damaging the Mexican government's reputation too greatly.
Mexico needs to do much better to have any such chance. To start with, they'll have to defeat Scott's landings at Vera Cruz AND win one or more major victories in the north, thereby retaking some of the lost territory, or their bargaining position is still very poor.
Agreed again. Mexico needs victories and keeping Scott bottled up on the coastal plain, perhaps by having an earlier version of the 1862
Cinco de Mayo victory, which would leave the US army at risk for yellow fever could possibly do the trick. As it happened, Scott got his army into the relatively healthier uplands just in time.
Bill