That's not the way things work in the US. First, the president is not a figurehead of the legislature. He's the head of a separate branch of government called the executive... which also contains all US agencies: DOJ, NASA, HUD, CIA, BLM, SEC, etc.
He nominates judges, and the legislature (which is its own branch) decides if they agree with his picks (by voting in the senate).
Similarly, he nominates people to oversee each division in the executive branch, and the senate confirms those people also. The people he nominates are in charge of their division. He can't go to the DOJ and demand they drop cases... that's a crime called obstruction of justice.
Constitutional cases are heard by the supreme court (but Meng's case isn't one of those).. but if the president does not like their decision, he can't do anything himself about it. There are only two ways to change the constitution: by getting 2/3rds of the 50 states to agree to the change (very hard, long process that usually takes much longer than the president's term.. rarely attempted); or by nominating judges to the supreme court who favor a different interpretation.
On the second part, there are a few road blocks too: whoever he picks must be confirmed by the senate. Once they are confirmed, it's a lifetime appointment. So if you look at the current court, you can see the current judges were nominated by 5 different presidents over 30 years. Since he needs to wait for one of these judges to retire, the president's picks will always be a minority on the court. He has no way of knowing when or if someone will retire. He'll only get a couple of chances to nominate a judge during his term (presidents serve max 2 terms of 4 years).
The legislature and president can change laws (sometimes those changes are the result of court decisions), but they don't change the court's decisions. Just their future decisions. Also laws can't override the constitution. And there are limits to the laws they can pass.. they can't for example make things illegal retroactively.
Also, I should add, the president can be imprisoned for violating the law. If he obstructs justice, he can impeached by the legislature and removed from office. He can then be prosecuted for the crimes he's committed. It's very unlikely though, because in the past, the vice president (who replaces the president after he's removed) can pardon the president for his crimes.