The key principle is Article 3 which states

1. This Regulation shall apply:

(a) to passengers departing from an airport located in the territory of a Member State to which the Treaty applies;

(b) to passengers departing from an airport located in a third country to an airport situated in the territory of a Member State to which the Treaty applies, unless they received benefits or compensation and were given assistance in that third country, if the operating air carrier of the flight concerned is a Community carrier.

So UA should not be denying you compensation and claiming the Regs dont apply to them because they are a US carrier.

You wouldnt be able to sue in the US courts because the US courts would not have the jurisdiction to adjudicate on European Law. Instead you can file a claim in a court in the member state from which you departed or you can contact the European Commission to investigate a matter.