Beheadings have been common in Mexico from the cartels. That isn't evidence.
The link between the Quds and the Mexican cartel is also a joke. That didn't hold water and it was pretty much laughed at because of how shoddy the links were.
No doubt they have some people in Mexico, Canada, and the US. Hell, a couple years ago a couple Iranians were arrested for trying to make car bombs down the road from my house. It made national news. It is going to happen.. but it'll take them time.
Hezbollah is funded first by the poppy plant, second by the US (indirectly) and others, including Iran, Saudi Arabia, Iran, Afghanistan, Jordan, etc, name them all. They all have ties together.
You make a point but what do you want done about it? Securing the border can only go so far against an enemy we can't easily identify.
Based on my intimate knowledge, we will not go to war with Iran. We have no incentive or reason. We don't have the money, the heart, or anything else. Iran's empty threats are worthless and laughable.
Sure, they might have some on our border.. a few dozen to maybe a couple hundred. But you're going off into the extremes about this.. tone it back and really think about what they want to accomplish.
The reason we have our army over there is to fight them there, not here. Read up on Michael Yon, or Generation Kill to get a perspective on how these people think and what they'll do. They're not coming here to fight us.. they're fighting us there because they don't know anything different and it is considered "fun" for them. Completely different culture.