The dunstan should do alright outside in Seattle. Do you think your location up a hill makes a positive difference for some cold sensitive plants?
I know of a dustan citrumelo growing and producing in Nottingham, England. And that’s not exactly known as a mild part of the UK the way southern England is.
A few dunstan i know about breeze through the winter in the Willamette Valley. Strangely, mine hasn’t produced any fruit yet. Despite being a grafted specimen on flying dragon rootstock and nearly 3’ tall it hasn’t even bloomed. The grafted yuzu I got on the same day has already made one small crop. There is a fairly decent size dunstan tree at One Green World, but I don’t know if it has produced yet.
Here’s a pic of the dunstan citrumelo in Nottingham. They describe it as a swingle crossed with a grapefruit.
Of all the testimonials I’ve read about ‘hardy’ citrus, the dunstan citrumelo gets the highest praise. Evidently it has the least trifoliate taste. Of course there can be significant variability from one tree to the next, so only time will tell.