You made several good points about Canada being a likely point of entry for terrrorists.
I would like to make a few points about the possibility of Mexico as a point of entry.
At first glance, many middle easterners have similiar skin tones/hair color to hispanics. That fact might lead them to believe they can "blend in" better on the southern border.
Many more illegal entries are made on the southern border, stretching the capabilities of all available enforcement agencies so thin the capture of terrorists would be less likely.
Human smuggling is more highly developed and organized on the southern border, so it would be easier for them to pay someone to smuggle them in. The human smuggling network on the Mexican border has links to every major city in the US.
Though I believe you are correct that support groups are more available to Muslim terrorists on the Northern border, corruption is so widespread in Mexico, even at the official level, support could easily be bought in Mexico and in border areas on the US side.
I admit I have never been to the Canadian border- but have you seen the remote areas of the Mexican border? There are many, many miles of remote areas of the Mexican border with little to no fence or markings. As far as ease of crossing the actual border; the millions of illegals that have already crossed are pretty good proof that it isn't hard to illegally cross the Mexican border.
If I were a terrorist, I would come in through the Mexican border, where crime and illegal entry are much more common, in all the chaos on the southern border I think I would be more likely to pass through unnoticed, and would bet I would be able to buy my way out of trouble if something went wrong.
I have canoed thru Big Bend. I can't imagine a more remote location in North America.
All of your points about the Mexican border are well taken. But my point is that:
1. We automatically assume any terrorist incursion will come over the Mexican border.
2. Yet, the Canadian border is every bit as vulnerable (and perhaps even more approachable) in this respect.
Thus, if it is terrorism we are really concerned about, we need to be equally concerned about both the southern AND northern borders.
If, instead, it is illegal immigration, then only one border is a problem.