Many thanks guys- I'm astonished by the responses! So useful!
Firstly- Just flying into one airport- that's genius! How that idea eluded me I'm not sure, but yes, it does seem eminiently feasible, and knocks a good £300 off the cost of the flight (and potential savings on car hire) and, though I'm reluctant to ever leave the coast(!) you've given us some great suggestions for the route back. I was not delighted to hear that my compatriot will be flying via AUSTRALIA for cheaper than we'll be getting there, but hey, that's the price of flexibility in booking a little later.
I will be moving heaven and earth to see the Fighting Banana Slugs and Eugene will definitely be getting a visit- can't remember the last time I didn't enjoy a 'liberal enclave!'
Finally apologies for suggesting that the map was a little bare above San Francisco, that was of course, because I am an idiot, and was using the map in my mind, which is a little unreliable at the best of times, especially regarding places I've never been to.
Please keep the suggestions rolling in....