WOW great thread loved the photos. I bet dealing with cutting into the hillside really put a sting on the budget but honestly it blends in pretty well I think and in the summer you might get the added benefit of a cool wall.
I noticed you didn't tie the foundation drains in with the roof drains, is there a reason?
Also you might want to invest in some downspout filters - screens, won't take much to fill up those pipes.
I tried to download and view the 2 PDF files for your dry well but didn't have much luck.
I was really interested to see your thread as I'm looking to build in Sumner and the > then 2000 sq foot rule triggers the drywell the city says so seeing what you built was great info.
Did you have to do any soil testing or sampling? or was it cut and dry this are needs X per Y here you go.
Thanks for sharing and the info looks awesome