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These are Spindle Galls.
This article has more information.

https://www.gardeningknowhow.com/plant-problems/pests/insects/spindle-gall-treatment.htm

I believe this is Plum Pocket disease.

Fungicides can help.

https://www.extension.umn.edu/garden/yard-garden/fruit/integrated-pest-management-for-home-stone-fruit-growers/plum-pockets/

https://www.gardeningknowhow.com/edible/fruits/plum/plum-tree-diseases.htm

Unfortunately we make it a policy not to recommend brand name products, but this article gives a few recommendations for effective chemicals at the very end: http://www.extension.umn.edu/garden/yard-garden/fruit/integrated-pest-management-for-home-stone-fruit-growers/plum-curculio/index.html

That appears to be a ladybug larvae, but since it is eating the fruit it leads me to believe that it is its close relative, and hybrid cousin... The Asian Ladybeetle larvae.

Fertilization during the first year is appropriate, but wait until you see some new growth in the tips of the foliage, indicating that the tree has recovered from the planting process and begun to put out new absorbing roots. Be conservative and use a slow release natural/organic fertilizer product. Harsh chemical fertilizer can burn and cause dieback. Mulch the soil surface to help retain soil moisture and enhance soil organic matter and benefical biological activity.

Here's an article that may help:

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/best-times-to-apply-fertilizer.htm

"As a rule, avoid fertilizing new plants until they establish. It can cause a burst of growth that makes the plant weak and leggy."

Can you include a photo? I have a feeling that a fungal infection may have had something to do with the fruit dropping off. A photo can help me to make a better judgment call.

This article will give you some information in the meantime: https://www.gardeningknowhow.com/edible/fruits/plum/plum-tree-diseases.htm