I ended up using the 1% milk, butter, and a bit of flour method.
BTW, when they say “make sure the butter has cooled but not resolidified”, they mean it. I put it in when it felt cool, but it wasn’t. It went back to butter bits the instant it hit the cold milk. So I scooped out the solids and did it again, waiting until the melted butter was fully cool but still liquid. Took awhile, but it stayed together with lots of whisking. And did exactly what I wanted it to do in my soup - made it slightly creamy.
I probably should have used the half-and-half, but I need it for my coffee in the morning. 
