Does the US have some kind of treaty in place with European nations to enforce a US court subpoena to hand over evidence or compel testimony in a trial of a crime of dual criminality?
NO. We have extradition agreements with some nations to return a criminal but not a witness. To return as a witness is voluntary as it should be. One must honor an order to appear in court in the USA if you are here in the USA. How would you feel about an order to appear in court in Iran, Cuba, Equatorial Africa, etc?