Could the white tip be new growth? Could the white be fungus? 2 inch pot likely dried out too fast in between watering. How did the root systems on the plants that are recovering look when you repotted them? How does this compare to the roots shown here in this post?

Keep this plant somewhere that it will be in good light, with warm temperature, with good air circulation, yet absent cold or hot drafts and where you can observe the plant daily for signs of recovery or decline. Just remember it is not always possible to arrest the decline of an ailing plant. Jades are pretty tough though and you should be able to revive this one.

I haven’t checked the root systems of the successful repots because I’m afraid to disturb them! Maybe I’ll try and take a peek this weekend..

I was not really asking you to disturb the healthy one, just thought you might have noticed the roots while you were potting them. I think it will suffice that you simply give the weak plant the best growing conditions possible, no need to compare roots between plants. You might want to isolate that weak plant, just incase, not that I think it has anything catchy mind you, simply a precaution.

I am still puzzled about the "white tips". What is your gritty mix made of?

Healthy roots are normally white to tan, and very succulent.

Jades are pretty forgiving of root disturbance, but that doesn't mean they like it. My guess is that, by repotting, you probably set your plant's reestablishment back at least 2 weeks to a month. Jades like to be repotted just prior to the summer solstice.

Al