I just thought 'locals' was kind of an odd word to use
How close and ready was the US army, when it came to intervention? Houston had ~850 men. There were 4000 Mexican troops in TX that hadn't been involved at San Jacinto. 2500 of them were ~50 miles away from Houston's force. Some of the Mexican generals (one of them Urrea) wanted to strike at Houston's force. The only reason they didn't was because the general in charge, Filisola, was unsure of his authority (being a foreigner), and was worried about attacking the people who held SA captive (fearing he might get killed in the process). So he ordered a general retreat of all Mexican forces out of TX. If SA had been killed/not captured, there wouldn't have been much to stop the Mexicans (who were pretty damn close) from stomping Houston. Would the US army be in place, in strength, and in time to stop that?
this is the same Mexican Army that took serious losses marching from central Mexico to the Rio Grande, so their logistics, such as they were, are in a tattered state and food supplies were a problem as they have by the time they get to the Harrisburg area marched past the area of principal settlement where there was any supply of food. So the retreat was also partly from logistical reasons.
But yes, Filisola did accept the order to retreat. Granted that order was issued under duress, and most European officers would have ignored it. If memory serves those 2500 men were in two columns as well with some distance between them. As Houston's losses were tiny, he still had a pretty decent force, now equipped with all of the battle field loot from the force that just got wiped out, most importantly, all of the powder and shot from it. He also had a decent force of cavalry to harass the Mexican forces that remained. 50 miles without roads is at least 3 days march by this force based on its speed so far in the campaign, perhaps longer.
When led by decent officers (not Fannin), the Texans regularly outfought the Mexicans at San Antonio (for example) and with Goliad as an example they certainly wouldn't have surrendered. They had water and food, could not be outflanked, and Mexican cavalry was in bad shape (mounts were worn out) so it would have been infantry vs infantry with the Texans having a defensive advantage. Not a sure thing at all, as you have to expect some more attrition on the Mexican side just getting there (food supplies low still). In fact the possibility of a serious Mexican defeat and disastrous retreat south is not unreasonable.
Filisola was right. His President was captured, a good chunk of the army destroyed, another large chunk dead at the Alamo and march to it, and he had no supply base and foraging was getting difficult. Better to retreat and come back later (which the Mexicans did do... in 1842). Without a head of government there is also a real consideration of what the proper thing to do is.
As to the US Army... it was on the border, and was a couple of thousand men if memory serves. This is an extremely experienced force (Seminole Wars) with West Pointers as junior officers, good weapons, firm discipline and commanded by another personal friend of President Jackson. One wonders what the actual instructions were. But if Houston's army is wiped out it can be in Harrisburg very quickly (200 miles, and this is a highly professional force so figure 10 days, maybe as few as 7 based on its typical performance in the Florida swamps.. similar terrain to East Texas by the way, including the mosquetoes), and if the Mexican Army pursues the refugees of the Runaway Scrape, that US Army can be on it very quickly indeed.
As to locals.. yes, by Texas standards they were locals as most of the settlers had been there at least 10 years or more. As before they arrived the Tejanos were in the few hundreds, not many could really be called native except the Indians. As about as many Tejanos fought against Santa Anna as supported him, in a lot of ways the Texas War of Independence was a civil war too.