I would give up on hoping to get much turfgrass to grow under them. It can be done but with constant needle raking and removal and adjustment of soil Ph.
I would look at the many nicely textured ground covers that will thrive under the trees. Ivy in all its forms would be a possibility but so would many other types. Acid loving, shallow rooted plants like azaleas are another possibility if suited to your climate and you can keep their roots moist and not soggy and mist them once in awhile. Drip irrigation will be perfect for the watering and misting.
Take a look at plant selection guides for your climate zone. Oregon is a pretty adaptable growing climate as I remember from many visits. I almost moved your way and may still at some point.