This is, as near as I can tell, good news.
My Mom is, amazingly, back in her assisted-living apartment as of this afternoon! The hospital managed to stabilize her daytime oxygen level with the nasal cannula -- and her sleeping level with a semi-coverage mask so she gets enough oxygen breathing through her mouth.
I found out by going to the hospital to see her and being told, "You just missed her." This got me to my target heart rate in a hurry but the nurses were quick to explain. Her senior-care place is not far from the hospital and I arrives just after she was settled in. She seemed pretty chipper. It was quite a relief after the fairly grim information I'd received over the weekend.
The nursing staff at the home -- overworked though they are -- are up to speed on changing the mask and the ones I have met seem pretty sharp.