Undaunted, I tried again. Twice. Here's what happened.
In the very first attempt I used heavy whipping cream and did not see any of the required curdling. My Internet reading suggested that a better choice would be half-and-half because less cream would allow more curdling opportunities.
Let me note here that the first attempt did look a bit grainy, which I suspect was curdling. It never got thick, which I think was because I put in too much wine.
So for attempt number two I used half-and-half. I used the same amount of sugar (2 tablespoons), the same wine, and just a little more of the spices.
I definitely saw curdling but it didn't get very thick at all. Again, I think I put in too much wine even though I used less than in the first attempt.
It was served in a bowl garnished with a few berries. It was thin enough that the berries sank!
Overall it was tasty and I would call it a success. Creamy and slightly sweet. The spice flavors came through and were pleasant. Light, cool, and refreshing; just right for a hot day.
When the custard sat in the refrigerator for a few hours, it separated. The curds on top were thicker and still tasty. The liquid below tasted like thin wine with a bit of spice. Not bad but not exciting. I suppose I could put it in oatmeal or bread for flavor.
One guest taster suggested that the berries become the main part of the dessert and the custard become the topping. This gave me a focus for the third attempt.
So one more try! Honest! I had to call it "Custard's Last Stand" to convince my family there would be no more attempts. (They groaned.)
I used all the same ingredients as before except I forgot to add the spices at the end. I measured the wine, putting in one tablespoon at a time, until I saw curdling. It took 7 tablespoons to accomplish what I thought was a decent amount of curdling.
And then I was called away to take care of something and completely forgot that the custard was sitting on the stove on the (thankfully) very lowest heat. I don't know how long it sat there. When I found it, I hurriedly stirred it, put it in a bowl, and popped into the refrigerator, crossing my fingers that it wasn't ruined.
In fact, it was excellent: thick and creamy and flavorful with very little separation. Certainly not the "curds and liquid" layers from attempt number two. Really just a thicker part on top and a thinner part (both with curds) on the bottom. The curds were bigger but still not a texture issue.
The missing spices were not missed. I used the custard as a topping on a mixture of strawberries, raspberries, and blackberries.
The berries were sweet and a little tart. The custard was creamy, a little sweet, and a little tart from the wine. The crunchy topping was, well, crunchy to offset the creamy and bring the flavors all together.
An unqualified success!
We had the same dessert several nights in a row and thoroughly enjoyed it each time. Very refreshing.