It would be an FAA decision on prosecution and a US Court decision on whether or not an offence had been committed. They are not going to lose any sleep over convoluted arguments like that.
EASA and the EU are not states and have no legal jurisdiction, nor are they ICAO contracting states. EU States are required to enact EU law into their own national law. The extra-territorial effect of the UK legislation, to which I've already provided a reference, does not extend beyond UK reg aircraft so no offence will have been commtted under UK Law.
UK Law, reference again supplied, does NOT say that a license is invalid if it is not accompanied by a valid medical certificate. it says that the privileges cannot be exercised.
The US has decided that it will allow the holder of an FAA 61.75 Airman's certificate to exercise the privileges of THAT certificate subject to the foreign license being readily available in the aircraft and the existence of a valid medical certificate issued in accordance with Part 67 CFR OR one issued by the foreign state.
