Certainly, not every Muslim is a “plant” or member of a sleeper cell. But, the numbers of them who are are much greater that zero. The Left has bet the farm that they can import enough voters to regain power and that radical Muslims, drug lords, and the free stuff hoards will gladly take all of the handouts and not cause the Left any trouble. They are fools, of course, but it’s probably the only option open to them.
Loyalty oaths for ordinary life actions were long ago wiped away by societal pressures. However, I believe every person in the US has some form of conscience loyalty. I do not believe that the Muslim/Koran dictates allow/permit loyalty of spiritual AND secular to be separate from Koran established dictates. If and when push comes to shove I would not bet on any person holding to the Koran to hold to the US Constitution. I think Tom Jefferson had this figured out many years ago.