Mexico is a sovereign nation. The United States solving the problem is both a violation of Mexico's sovereignty and a very paternal reaction that denies the Mexican people their agency.

I dont think that the Mexican people are actively choosing this. I don't think they want it either (drug trade). So if our goals are aligned, why is it considered a violation of their sovereignty and not just helping a neighbor out.

If my neighbor has an issue on their property, and they ask me not to intervene I will not cross onto their property to help, even if I think I should (unless you know, murder or whatever). But if my neighbor asks me to help, we should work together. It seems like we can work together in a more overt, positive, non-covert CIA way. I'm not saying this would work for certain, just that aybe we should try something new.

You say that like the US is not their biggest customer of illegal drugs.

That's true, but Mexico also exports huge quantities of narcotics to Canada, Europe, and other countries. Who is responsible for solving the problem?

And regardless of responsibility for causing or solving the problem, we absolutely need to improve border security. Physical fences aren't very effective in most areas but better sensors and more human guards could do a lot to reduce smuggling and human trafficking.

And has spent billions attempting to limit their purchase & international movement. Maybe it should continue to be a joint effort rather than finger pointing that one side isn't doing enough.

which means the US can dictate Mexican policy?

The claim is not that a border wall would keep violent cartels out of the US, but that it would drive down the drug profits fueling Mexico's violence.

I'm not sure I agree with this claim, but "this puts a fence around it" seems like a complete misunderstanding of it.

If walls worked, our prisons wouldn't be full of drugs. These systems will by bypassed and corrupted easily by well-funded drug cartels.

Sure, but then we're not "putting a wall around the problem" either. Like I said, I'm not agreeing with the claim, but your objection contradicted itself

How does the US "solve" this problem? I don't see any direct way of doing this without some sort of military intervention. Is that what you are suggesting?