This was a muslim terror attack.
No, it was an act of terror committed by a Muslim. There's no Muslim, Inc. He isn't an agent representing Islam.
Could it be
a} that Americans have not made any distinction between Muslims and other religions when screening for entry?
No. He was born here. His parents, Muslims who weren't, assimilated. What are there a billion and a half Muslims world wide? And of that how many are fanatical terrorists? A relatively small percentage, being fought ideologically and physically by, it should be noted, other Muslims.
b) that Muslims are in fact much more likely to seek to pursue their aims through terror than other religions?
I'd be interested in a breakdown, by percentage, and a comparison to those who commit acts of terror for other reasons as well. Christendom has a pretty bloody track record, though we've settled in nicely. Islam, like our faith, has a huge following. Hinduism would be a distant third, but still sizable. I know how it went in India and Pakistan, but I don't know how it shakes out these days.
c) that many Americans choose to love Muslims because it fits their ideal of a liberal society, even though they know that Muslims themselves are far more likely not to reciprocate?
I don't accept your premise that a) you have to be a liberal to love others, that b) this is about loving others, or that c) Muslims are inherently more anti social. Extremist anything looks crazy and acts crazy.
d) If your constitution requires you not to discriminate against people on religious grounds, does that extend to non-citizens who are applying for entry to the USA?
Immigration is a process of discrimination when you think about it. It has to be. What our principles should require of us is that our standards be rational and consistent.
e) What will happen if Muslim immigration to the US goes unchecked?
No immigration occurs without checks. And most of our immigration is coming from Latino populations who bring their own potential for problems
Is asking such questions wrong because it is disrespectful to those who have died?
I think using the nightclub incident as fuel for it is a bit knee-jerk, given we haven't really established much about him other than he was born and reared here, was allegedly abusive within his brief marriage, and hated homosexuals.