The thing with the kitty treats when they come back in is working quite well: often I only have to go out in the back yard and clap my hands to have the kitties come running. That's a re-assuring thought. We do keep the back door open when they're out, and often they'll pop back in for a bit and then go back out again. Still, having the door open in the evenings is not going to last very long in this time of year, so we'll have to think of something clever.
The black-and-white cat has made a re-appearance twice more. One time he chased Mikan, who retreated to the house. And then Yuzu popped up from behind the bicycles and he found himself surrounded... I defused that situation by chasing him off, which was fine with our kitties.
But today, he chased Mikan again into the yard and followed -- and again found Yuzu there too. The tide of battle was quickly turned and the two of them chased the intruder off. Yuzu went after him, and she hasn't come back yet, so I guess she's (trying to) teach him a lesson.