Moving on to the geraniums, if weather is wet and cool, botrytis can cause the symptoms you see. Over- or underwatering can also cause yellowing leaves, but the wilting flowers make it sound more like overwatering. Make sure drainage is adequate, and water less often. Fertilize with a balanced fertilizer to rule out nitrogen deficiency since geraniums are heavy feeders. The symptoms on the rhododendron suggest dieback, although the leaves usually turn more of a reddish-brown.
Rhododendrons prefer rich, peaty, acidic soils.
I have a big plant of hardy phlox. When we moved here in 1948, the plant was here and has come up every year since. It is white and smells so good. My mother always sprinkled it with sulfur. Sprinkle it well down in the bush.