Sorry to hear about your problem. I don't believe a dogwood tree would die in one year from contracting this disease or be threatened by losing some shade. Here in central VA most dogwoods are growing in full sun, some 20-30 years old and others younger. Wild dogwoods grow here along the forest's edge and receive a good amount of sun.

If you have the 'Florida' variety and contract a disease, you'll have to spray them with a fungicide such as Daconil or liquid copper approximately every 10 to 14 days from now till mid-June. 'Florida' is "resistant" to many diseases such as powdery mildew, anthracnose, and canker, but you'll notice the word used is "resistant" and doesn't mean the tree won't encounter the disease.

I lost two dogwoods in the 'Florida' variety and have one red dogwood of this type that I'm babying by spraying and praying. I decided to try the beautiful 'Kousa' variety of dogwood that's far more hardy. Look for a single trunk type. When 'Kousa' reaches maturity, it's absolutely elegant AND more hardy than 'Florida'.

I'm surprised that your part-sun/part-shade perennials died from exposure to more sun. They CAN take a half-day of sun with the addition of more water. What variety are they?

Are your neighbor's shrubs healthy? I'm wondering if you should have a soil test since all your plants are dying. In Fairfax county you can get the kits at the library, so check there wherever you are. Several years ago the cost was $7. You mark the form for what your soil usage is, follow their directions for collecting a soil sample, and the VA Tech extension service sends back a report and recommendation.
A lot of shade perennials are moisture lovers, and a tree plus shrubs can suck a lot of water from the soil. Weekly deep watering is supposed to be better. One way to check for an inch is to put a cat food can out where you are watering. When it is full, you've watered an inch.