Finally had a chance to research the solitary bees in my garden. They are Wool Carder bees (Anthidium manicatum) named after the female's habit of scraping the hairs off fuzzy plants, rather like carding wool.
The site I found didn't mention their machine-like way of hovering which I find remarkable, but did say that they're prevalent in gardens and that smaller-flowering Digitalis (I have D. lamarckii) are one of their favorite plants. Also that they will aggressively defend a small group of flowers which we have most certainly observed.
In fact, I hesitate to mention this... Brandon decided to capture one of the bees as we watched one of them terrorizing the others. He had done this many times as a boy, but I was unclear on the process when he brought out – of all things – a honey jar, quoting Bart Simpson: "the ironing is delicious."
He effortlessly closed the bee in the jar while it fed on our salvia. In no time, inexplicably to me, it was looking near dead. He had to explain to me that a cotton ball soaked in alcohol (which I hadn't noticed in the jar) will kill an insect – somewhat humanely. I didn't much like this and saw little point in capturing when you can photograph.
But as it turned out, seeing the actual bee up close made identification much easier. They are much fuzzier than they appeared while hovering around and there are 3 tiny spikes on their behind which one could only photograph for ID purposes with great difficulty. So, information gained, but a little life lost. I did spy on a couple of them performing 'the act' so hopefully we haven't altered the balance of nature too much. Perhaps we could make a bee post to encourage more.