Originally when I was booking, the agent insisted that my JFK to LHR flight be on American metal. Then I made a couple of tweaks and added on Las Vegas, and the person making the changes didn't question it being AA metal - codeshare was good enough.

Yeah, bit of backtracking - combination of maximising tier points for status, but also this is part of long term travel so these are all places we wanted to go anyway. So I just routed them to suit the time of year we are going to each place, and also maximise status earned - worked out pretty well for us.