The "structures" on the leaves are galls. They are a reactive tissue growth, typically in response to some insect issue but harmless to the tree (if somewhat unsightly).

Unfortunately, the pics of the fruit are not clear enough for me tell what might be the issue there but whatever it is, it is unrelated to the galls. Without any other confirmation of what it might be, I'd remove any affected fruit as well.

Sorry about the blurry photos - the branches were swaying in the breeze. It is pouring rain now but I will try to get a clear photo later. The infected green fruit is covered with the yellow stuff, and appears swollen to 2-3 times the size of the unaffected (yet) fruit.

Interestingly the leaves with galls appear only near the infected fruit, so I thought they must be related.

Sorry to be delayed with a better photo.

This fruit was like an inflated balloon when I cut it open to see if there was any kind of pest inside - hollow except for the rather mushy, but at the same time partly desiccated looking (kind of stringy) flesh. Normal fruit for comparison - the infected fruit was at least triple in size.

gardengal48 (PNW Z8/9) Any other thoughts with the new pics? Anyone else?

The good vs bad plum pics remind me of baby shriveled squash that aborts when it's not properly pollinated. Maybe the bad plum had the same fate? If you didn't find frass/brown rot/maggot inside when you it cut open then bug infestation such as a curculio can be ruled out.