Update: I searched online for help. I found one source where someone else had the same problem. They were told by other members of the forum that the mixture could not be saved and should be discarded. Another forum advised a person with the same problem to stiffen the mixture with confectioner’s sugar and pass it off as intentional (ick).
After being online for about 20-30 minutes, I went back to the kitchen and took the white soup out of the refrigerator and put it back on the mixing stand and whipped it. It did not reinflate to the original volume but it did fluff back up about 50%, and that’s pretty good. I whipped in more butter and also the cooled, melted chocolate, and whipped it a couple more minutes. It was great! It did not have the total volume of properly mixed buttercream, but it was smooth and tasty, and it formed well.
I used a large-size decorator’s tip and put swirls on top of all my cupcakes, and they were great. I got tons of compliments (they were for a birthday party). So, crisis averted! And I learned to *let the meringue cool* before beating in the butter!