Yes, if your delayed flight is eventually cancelled, you can claim compensation if they fail to bring you to your destination on time. For the most part, airlines are supposed to give you an alternative flight in case of a flight cancellation. If they do and you are still more than 3 hours late, you should be able to claim compensation.

Your right to compensation becomes active from the moment you reach your final destination. If you waited at the layover for more than 3 hours but the airline still managed to somehow get you there on time, they are not legally required to pay compensation. Remember that the full trip should be booked under the same ticket.

Yes, your citizenship doesn’t affect your right to compensation. As long as you meet the requirements set under EC 261 in terms of departure location or carrier origin, you should be able to get compensation regardless of where you live.

This depends on the reason behind the technical delay. If it was some hidden manufacturing defect that the airline couldn’t have caught, they might push for extraordinary circumstances. However, if it’s a routine issue that could have been caught during maintenance, you might be able to get compensated based on the distance of the flight.

Whether you purchased with miles or points, it doesn’t matter; the UK or EU airline is paying you for your inconvenience. Since frequent flyer miles are still, in a way, money, EC 261 gives you the right to compensation without any issues.