Airforce, illegal entry into the US is a crime even though you also quoted a civil penalty for it.
8 U.S.C. Section 1325, I.N.A. Section 275..."For the first improper entry offense, the person can be fined (as a criminal penalty), or imprisoned for up to six months, or both. For a subsequent offense, the person can be fined or imprisoned for up to two years, or both. .."

Airforce you asked,
"...If President Trump prevails in court on this matter, what would prevent President Kamala Harris from declaring a "gun violence emergency" and outlawing the possession of Ar-15's? In fact, what would prevent a future president of nullifying the Second Amendment and confiscating all firearms? President Lincoln used a state of emergency to suspend habeas corpus, what is to prevent a future president of doing the same?..."

What will prevent any president from doing this the 2nd Amendment and militiamen.
"A well regulated militia being necessary to the security of a free State, the right of the People to keep and bear arms shall not be infringed."

"All laws which are repugnant to the Constitution are null and void." Marbury vs. Madison 5 US (2 Cranch) 137, 174, 176, (1803)
The United States Constitution is the supreme law of the land.

Gun confiscation will never be tolerated by gun owners and militias. Without the 2nd Amendment and armed citizens the rest of the constitution is not worth the paper it is written on.

Militiamen realize, as did the founding fathers who threw off the tyranny of the British; that our rights come directly from God himself who is a higher authority than any government. On April 19, 1775 when government troops marched on Lexington and Concord to begin gun confiscation it was militiamen who stopped them firing the shot against tyranny that was heard around the world which started the United States of America. Militias would stop tyranny again.

Gun ownership is not negotiable.