Hello there Houzz Community!

I have a large 30+ year old Jade plant that was bonsai'd by the
previous owner. The plant has been with me for 2 years with no obvious
problems. My regular routine with it includes watering it every 1-2
weeks and the soil does dry out between waterings. I prune lightly every
few months. It is in a fairly low-light area (unfortunately). It was
overwatered once about a year ago when I was on vacation.

Over last 2 months it started to drop branches and normal-looking
leaves were falling off with lightest touch. The leaves have no spots
and there are no signs of mites or other pests on the leaves, trunk, or
soil. The pattern is the branches get soft, slightly darker, and
wrinkled at the spot where they detach before the branch falls off. The
most distal parts of the plant were affected first, but it has been
dropping progressively larger branches. The VERY large branch (1.5"
diameter) that fell off this week had a green-ish mold inside the
branch, which I've never seen before.

This is a picture of tree - plastic covered area is where branch just fell off, but this bough lost two other large branches distal to this within last two months. All the recent problems seem to be on this side of the tree, but old scabs on the other bough may indicate there was trouble before I got this tree:

This is the spot on the bough where the limb fell off:

So, I was worried that something may have infected the tree and was moving down the plant, so
I cleaned the branch with a 1:9 bleach solution and cut the branch off
1.5 inches below the mold/darkened area with a sterile knife. I cleaned the cut
area with more of the bleach solution. I was quite surprised to see the
entire interior of my plant is brown:

I
then cut into the branch that had fallen, and trimmed off 3 inches of
limb, and while it looks better than trunk, the vascular
cambrium/xylem/phloem still look brown. These two slices are 1 inch
apart about 3 inches distal on the branch from the rotted place:

-Does brown interior of the plant always indicate rot, or could some of this brown coloring be due to age/size?

-Should I cut through the trunk (about 4 inch diameter) below that main bifurcation and see if trunk is also brown?

-Should
I unpot the plant and further investigate the roots? If the roots are
rotten, should I remove them and start moving up tree with progressive
cuts to find healthy tissue?

-Should I remove the other bough from bifurcation and see if I can find a
place in the bough that looks healthy (not brown) to root?

-If I
find something to root, would it be better to dust the end with cinnamon
and let it rest/scab, or should I coat the cut end with rooting hormone
and then let it scab?

-If by some miracle the trunk isn't rotten, is it possible to graft healthy sections back onto the trunk?

-Should I remove the older leaves before rooting to make the plant lighter weight?

Thank you all for your help!

~Nolan