I'm sorry you never got a response. I'm just bumping your post so more people will see it. We have an orchardist (JONA) who has been a regular around here and is really good on questions like this, but I haven't seen posts from him for awhile.
The yellow "scale" you mentioned is lichen. It is just a sign that those branches are dead and so are now being colonized. The lichen didn't cause the dying, it just came afterward.
Tree is clearly dying, but for a big tree like that, that is a long process and parts of the tree may still be OK so far. I don't know enough to know what is causing it to die - probably some disease process.
Here's hoping you get a better answer!