This "balloon" navigates, not drifts, so any crossing of the Mexican border is purposeful. It has drifted across a swath of the US that is home to cancer and disease treatment research as well as key military installations. Traveling at 50,000 to 60,000 feet it has better optics than the satellites it communicates with. Further, it is equipped to provide telecommunications and internet access, per Loon.com. Does it also have the ability to sweep communications and transfer that info via the satellites it "talks" to?

Since it is steerable, what purpose could it have in Mexico, if it crosses the border? Will it get "shot down?" Who would that help?

Over the past few days, it has been tracked flying from Pittsburgh to Hagerstown, MD (proximity to Camp David and secret military installations as well as sattelite communications systems for guidance of government entities), down I-81 to Roanoke, then onward to Raleigh/Durham area, where it hung out for an extended period, followed by an excursion toward the coast (Camp LeJune and Norfolk as viewed from 60,000 feet), then a turn around to Huntsville and Dallas, home of the University of Texas/China deals.

Now it's about 100 nm from the Mexico border. Will it cross it? and why?

Loon is a subsidiary of Alphabet. Alphabet owns Google. Eric Schmidt, connected to North Korea and China governments, was CEO of Google, before resigning. He is responsible for assisting the Chinese govt in beefing up its on-line surveillance of people.