What about the house? Is there water sometimes being held against the foundation, that might even start coming in during an exceptionally rainy spell? Example: one winter when there was so much rain we had a pond start to form on the front lawn - something I don't remember seeing before or since - the house across the street ended up having a waterfall coming into the basement.

As far as the plants are concerned the growth and survival of the rhododendron shows that conditions must be fairly acceptable as most kinds of rhododendrons must have good drainage.

Would it be a good idea to drill a hole or two in the concrete wall?

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Definitely it can help. You drill near the bottom, and then insert a perforated plastic pipe in it. I have seen people do that when building retainer walls out of stone, which also can prevent the water logged ground from freezing and cracking the wall. By drainage, you solve that problem too.

you can also test the drainage using the old 5 gallon bucket of water test.