I know that area very well. Kinnikinnick is a native of the Pacific Northwest. Don't worry about amending that soil at all. When you plant, plant 1 gallon nursery grown plants using an offset grid pattern. I would rake off all of that bark first. Compost it.
For each plant you dig a hole no deeper than the depth of the root ball. Plant 3' apart on an offset grid pattern. You do not have to plant the entire slope. Chose the areas with the best sun and plant 3' apart in that area; imagine a plant at the points of an equilateral triangle as your base unit for your grid. You'll be making a 'sweep' of ground cover that will look thick and luscious in a couple of years. Think of the shape of paisley which is like a teardrop that gets skinny at one end and curves a bit. A bigger mass that sweeps into a thinner mass? Blob of paint pulled into a disappearing tendril? Someday I'll learn how to use this sketch program I just got, to add little drawings to answers.
The trick for you is to get the baby plants established. On a slope which that Kinnikinnick will love, you've got to make a little 'well' for each plant. Your soil is probably clay and clay is fine soil if you understand its properties. Yes, drainage is critical for Kinnikinnick, but planted on a slope the only problem you are going to have is getting water to the roots. Water usually washes right over the clay surface of a slope. What kind of slope are we talking about? Could you add a picture? I realize you've got your answer and Jurp gave you a great answer. I just know this plant and your area and I have to turn you onto a great product that is available to you in Everett. Call Sawdust Supply of Seattle. Talk to them about Gro-Co. This is the mulch you need to be using. We can talk about it later? I would replace all of my bark mulch with Gro-Co. You'll have to replace it more often as the soil organisms eat this and take it back into the soil profile mixing this decomposed organic mulch into the soil for you. I would not even use fertilizer if you were to use this Gro-Co around your Bearberry or Kinnikinnick. If you want to 'amend' your soil this is what you should do, get rid of the bark (do you have a green belt or 'natural' areas at the back of your property)? After planting your Kinnikinnick, making little terraced wells to hold the water back from running off down slope giving it time to seep into the plant wells...thoroughly watering for each plant, cover the top of the soil inbetween your plants with 2 inches of Gro-Co. (You are so lucky...I got spoiled with Gro-Co moved away only to find this is not made everywhere).
This is probably the best thing mankind can be proud of leaving behind. Grins, you'll see what I mean soon. This stuff is gorgeous (I am a Landscape Architect with callouses and dirty fingernails). Fine texture, dark taupe, like that of a rich soil, no sticks or stones or lumps, no weed seeds....no pesticide residue. A bit high in heavy metals would not use for my vegetable garden but most tap water has higher levels of heavy metal. In one week, your plants, all of them will look greener, healthier I am not kidding. It is a remarkable mulch. I won't use bark. I told clients if they liked that look they could find someone else to install the bark.
Not once did I lose a customer, I gained them like crazy when they saw what happened to the yards I mulched with Gro-Co.
I'll try to find a picture of a 'well' for planting on slopes. I also recommend a cheapo oscillating sprinkler to water. It allows the water to soak in without building up and flowing down slope taking mulch and soil with it.
Let me know what you find out when you call Sawdust Supply and learn what this incredible mulch is all about. I will never use bark or plastic for mulch. Never. And you have access to Gro-Co. Lucky you.