It sounds very much like a scale type insect. Since this tree has exfoliating or flaky bark, I'd remove a portion that has a population of these spots/insects and take to your local extension office for ID and treatment suggestions. The good news is that the insect showing on the leaf is the pupal stage of the lady beetle (ladybug), one of the best beneficial insects you can encourage in your garden. Will take care of those pesky aphids lickety-split! Can do a number on the scale as well. Whatever treatment you are directed to, make sure it is non-damaging to the beneficials.

I'm not sure what elevation you are at, but assuming the Marina will do ok at your elevation, this looks like a good spot for it. It needs open areas, will take part shade as long is its bright and open, so this looks like it would fit that bill well. I wouldn't hesitate trying the native Madrone there myself, if the soil is well drained. I would NOT add a Japanese maple next to it. They will grow quickly enough to compete for both light and water with the Marina, and though it likes it dry, an active fast growing tree like a Jap maple does not seem like a good fit to me for getting it established. I am intrigued by the thought of the Marina with the Stewartia, honestly. They are both known for their bark, which I think would make for an interesting pairing. The stump may work well for one of your hemlocks too, if you were interested in using it for that. Be aware though, Mountain Hemlock in my experience needs a fair amount of water to get it going, and really prefers more even moisture as it is growing than the Marina may like. Keep the two separated if you plan on supplementing the water for the hemlock. Once established though (which I find takes longer with this alpine than it would for most native conifers, especially if we have dry summers during the establishment phase), they should be fine with the drought, especially if they are growing on or near a "nurse log" which will help with that immensely. Maybe its just my love affair with the native dogwood... but I always like seeing dogwoods up against evergreens, lol. It would probably like the conditions you have their, and would be lovely with the white flowers against the colorful bark and dark green leaves, but I'd be a little worried initially about it overpowering the Marina, especially if you want it to be the star of the bed. Once the Marina gets its head up above the dogwood, it may be less appealing than when its a smaller tree with the combination of the leaves and flowers more apparent. I'm not at all familiar with the Oxydendron, so can't say much of anything about that. Seems to me similar enough in flower shape that if they bloom anywhere close to the same time period, I'd rather see them in separate beds, but close enough to where the flowers can echo each other without looking like you are trying to match them up. But it looks lovely, especially in fall color. On a side note... especially with the woods around you, I kinda get a red neck kick out of the "stump farm" look... I'd be awfully tempted to leave the stumps the way you have them, plug them with different kinds of (edible?) mushrooms, and garden around them for a while. Maybe bowl out the tops of the bigger ones enough to plant some ferns and maybe some ground dogwood in them or something... have fun with them rather than remove them. But then, I like the way these stumps look in the woods as they are decomposing. I kind of think of it as the cycle showing all its splendor right there in front of your eyes, lol. I've had good luck planting red huckleberries, spiny wood fern (Drypopteris expansa), licorice fern and a few others in these stump pots. I think with the new trees and the returning salal etc, you could have a really beautiful bed that way. One trick with this- use something really organic like peat moss or coir to plant these, or if you have it, composted bark. It mimics the decomposing wood, and the roots will be able to adapt easily to the wood as the potting "soil" and stump become one. Most of the usual suspects that grow on nurse logs in the woods will like this kind of organic soil too.

Zone 9 where? Really, the Sunset zone would provide a much clearer picture of your conditions. I live in Zone 9/Sunset 23. Clay soil can be a problem with Arbutus Marina. The lack of drainage makes them even more susceptible to Phytophthora. Even with good drainage, Phytophthora can be a heart breaker with these lovely plants. I have lost three growing in huge pots; one tree, in the ground (sandy soil), remains healthy (knock on wood). Arctostaphylos prefers sandy/rocky soils. Emerald Carpet also prefers acid soils. I haven't tried Emerald Carpet, but in my area (south Orange County CA), A. edmundsii ('Little Sur') and A. uva-ursi 'Point Reyes' are thriving in very bright shade/protected sun. The former beside a shed, and the 'Point Reyes' under Iceberg rose bushes. Not sure they would be thriving in full sun here. These are being irrigated with drip. Howards have not been easy in our garden. We have lost three out of four of the 1 gal. plants tried. But our watering system is drip, and these guys are known to hate drip. How are you planning to irrigate?

Thanks... I'll keep my optimism and hope for the best for my Arbutus marina. I've picked a spot in full sun (of course) with good drainage, etc etc. I've given up on the native Arbutus (at least temporarily). I've tried several times and have had limited success, though I've never had one die due to harsh winters. They always seem to die in late spring / summer / fall, for no apparent reason (which the native Arbutus is known for). The last one I had lasted for 4 years. It was a nice looking, healthy little tree and then, poof, it just decided to die in mid-July. I hope you have more success than I with that that one. I am (probably temporarily) content with looking at the huge healthy native Arbutus across the street in my neighbour's yard.