Hi,
The white spots...haven't seen them but I'd guess powdery mildew or mineral deposits, depending on where you live and soil conditions.
Might help:
https://www.google.com/search?...
If it was me I'd lose the entire rotten V section and go for at least 5 or 6 cuttings.
Hard to tell where those roots are, might be able to get more.
I've had good luck with my 50 year-old jade letting stem and leaf cuttings callous for a few days to a week and potting in a cactus mix soil with extra pumice. When I re-pot I add sharp sand and green sand.
Normally about 1" of stem with growing leaves in the center.
If there's no rot you can also just root the top 2" cut off a healthy branch.
Last time I did a major pruning I cut a dozen 1' branches, put them in an open cardboard box in my carport.
That was a year ago, and I'm still rooting new plants out of that box. They're hardy plants.