It's odd how emigre communities seem to "freeze" at the moment of their departure from their homeland, even despite further waves of immigration and access to modern communication. Brighton Beach's Russian community is living in this time warp of the 1970s to late 1980s even though they should know better. Because the US doesn't have as many old Muslim communities (the only one I can think of being Dearborn, Michigan), that factor didn't come to mind immediately, but yes, I can see that being a huge complicating factor.

As for how you test for the attitudes I listed, I think that you can't weed it out on arrival entirely. Obviously, if the person is coming from, say, Egypt, and Egypt's security apparatus says the guy goes to a Salafist mosque and is a member of the Islamic Brotherhood, then you have evidence (assuming veracity of the information you're receiving, which is a separate issue altogether) that would on its face deny entry. For others, though, they would have to agree to uphold the democratic principles of the society they're provisionally entering. This would then provide grounds to, say, deport the person if he/she, for example, harasses someone for being openly gay, or threatens an ex-Muslim with any sort of bodily harm, etc.

And yes, it becomes much harder when the people were born in the society, which is why being strict and selective with immigration is an important point to begin with. Obviously, the country should figure out how best to integrate immigrants as well; if you're going to invite someone in then you're taking on responsibility for them (the flip side of the immigrant's responsibilities).

I just think that it's clear that it's not just about terrorism, but also about tolerance, and someone who thinks that it's possible to let one million random refugees without having issues with that is beyond naive.