From one dog person to another, I am truly sorry for the sorrow you must feel in losing such a companion. That is never easy.
I have a few thoughts:
The wet area makes it tough for edibles. Most edible trees, fruts and nuts, like a well drained soil. Crab
apple is one exception.
This is a link to King County's
native plant site. I know you are in Piece but they should be similar
enough.
King County Natives One tree that comes back is the Pacific Crabapple:
crabapple It has flowers in the spring, height for shade in the summer, and foliage color in the fall, plus some fruit. (not sure how tasty.)
Another suggestion is to plant some natives trees that love water in a grove and then plant your bearing tree on the sunny southern side of the grove to enjoy. Also there are some native edibles that like water, so they could be grown under a
canopy to get the best of both, shade and treats. The website listed will allow you to search by all parameters seperately or filtered.
The memorial is a nice thing for your friend. I don't think that you have to go too deeply to worry about too much amendment. 3 feet should be enough for a small tree. The biologics will be well decomposed by the time the
roots need to be that deep. I would think even a few inches below the root ball of whatever stock you plant should be enough. The tree/bush will have some time to acclimate before it starts reaching out in a serious way. By then nature and the microbes will have had time to do most of their work.
Peace be with you until you see your friend again.