I’m researching a possible vacation for later next year.
The thought is to spend a few days in SAN before flying onto Hawaii for a week, then to PDX for a few days, before returning from SEA.

Hopefully I’ll be able to UuA to SAN (or LAX) and then I see Alaskan Airlines can provide the necessary flights before departing from SEA.

I’m not sure how to go about this. I could create a BA itinerary and a separate Alaskan one but it might be prudent to include the flight to SEA on the same PNR as the departure from SEA?

So is it possible to create just the one itinerary (over the phone, which I have to do anyway for my wheelchair user wife)?
Is there any benefit to booking through AA?

Thanks.