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You should have removed all the old potting mix. It's possible it's just sitting in there still dry. I really don't want to tell you to repot because it's a lot of work with a plant that size. I'm wondering if letting it sit in a bucket of water for an hour or two would ensure the root ball is soaked. Let others more knowledgeable than I reply.

You don't ever let it sit in the water which comes out of the inner pot, right?

Jades will loose old leaves, they usually dry up to crisp and eventually fall off (or you can take them off). There could be few at the same time, but usually just 1 or 2. Do you have just few or may yellowing at the same time?

Black spots could be oedema/edema, that is a disorder, not a disease. Those spots could be raised, like a pimple, often hard and you shouldn't try to take them off (they would have to be scraped, using bit of force, and would leave wound on the leave). Oedema could also look like irregular spots. If the black spots were oval-shaped and quite same in size, it could be scale insect but I do not think that's what you have. Powdery mildew could also have darker spots, but that is a fungal disease and often covers more of the leaf. It could also be 'fuzzy; - doesn't look like you have that either.

If the soil was hardened around root ball, it is likely because it is mostly peat that became hydrophobic. Not best for the plant, since hydrophobic peat is quite difficult to re-wet. Hard as it is, it repels water and regardless how much you water, plants roots can't get any. Since you repotted it with that hard soil on, all new soil will get saturated but roots will stay dry. It is better to get rid of old, hard soil and loose some, even lots of roots. Jade will regrow roots in better potting mix quite fast. The mix should be well draining. You can make it so by adding lots of sifted or rinsed perlite (or pumice if available). Mixing it in 1:1 ratio is recommended.