Tempo

Definitely 20. You can unzip in a summer. Like a lot. Or get a 15-20 and a very light blanket/sheet for the really hot nights

It really depends on a person, I'm very comfortable in my side sleeping big agnes 20 at 22ish in just my underwear. And it's a very roomy mummy bag

Look up spoon shaped bags. I have a nemo tempo 20 (which is synthetic) but they have down versions for more money. As a side sleeper it allows enough room to turn over in while also still packing to a compact-ish size.

I get the weight concern, but honestly for Washington trips, the Tempo 20's extra warmth is worth a few ounces if you're a cold sleeper. I used a heavy synthetic in the Olympics for years and never regretted it when temps dipped.

I opted for the tempo 20 for similar reason. The regular will be plenty. I am just shy of 6' and the long is a bit too long for me.

I'm from Washington as well and have the Nemo tempo 20 unfortunately haven't gotten to use it there. I've seen all the good reviews but I don't know if i'd reccomend it, it is quite bulky for backpacking and I've been a bit chilly in some temps that it claims to be good for. That being said I do like the bag itself and will still be taking it on my upcoming norway trip.