US Law overrides foreign law within US Jurisdiction. It doesn't matter what the British Nationality act says unless one happens to be in Britain.
No. At least in the area of citizenship jurisdiction, they coexist.
If what you claim were actually true, no one born in the U.S. to foreign citizen parent(s) would acquire the foreign parent's citizenship at birth (if the foreign parent's country's law provided for such).
We also know that what you've claimed isn't true as U.S. Secretaries of State have determined even after the 14th Amendment was ratified that those born in the U.S. to non-citizen fathers were NOT U.S. citizens. Secretary of State Frederick Frelinghuysen determined Ludwig Hausding, though born in the U.S., was not born a U.S. citizen because he was subject to a foreign power at birth having been born to a non-citizen father.
Similarly, Secretary of State Thomas Bayard determined Richard Greisser, though born in Ohio, was not born a U.S. citizen because Greisser's father, too, was a non-citizen at the time of Greisser's birth. Bayard specifically stated that Greisser was at birth 'subject to a foreign power,' therefore not "subject to the jurisdiction of the United States" within the meaning of the Fourteenth Amendment.
Source: A Digest of the International Law of the United States: Volume 2
The historical evidence clearly indicates that even after the 14th Amendment was ratified, a child born in the U.S. to a non-citizen father was determined to be born subject to a foreign power, did not meet the 'subject to the jurisdiction' requirement to acquire U.S. citizenship at birth, and was therefore not a U.S. citizen at birth.
“But we will not assume the exclusive right of saying what the law and usage is. Let us appeal to enlightened and disinterested Judges. None is more so than Vattel.” Jefferson