Well, she's home
Both she and the mother seem extremely healthy, and the mother's temperament is fabulous. The owner brought her in to the room where we were, and she calmly investigated us with neither fear, skittishness, nor was she demanding.
The kitten spent the (hour and a half) drive home laying next to my son's legs, looking like a statue of a kitten. At first she was showing some distress, to be expected since she's never been outside before, much less in a car, but she settled down quickly and just laid all stretched out with her chin on her front paws.
She's scared now, but I expect her to come out fairly soon. We just got home 5 minutes ago.
She is so soft, she feels like spun silk
ETA: 10 minutes later, she is now perched/cuddled on my ankle, which is crossed and resting on my other thigh. So she's sort of on my lap, but in a very creative way