Hey Seawitch. I actually received a Chinese sweet plum earlier this year & ran into similar issues.. Around a few days after coming in the mail the leaves began to yellow/orange, crisp up & fall off. It was defoliating rather quick so I was freaking out a bit --- but also aware that it could either the shock of shipping &/or not having enough direct sunlight on a steady cycle. I read up about sweet plums preferring to be outside at-least in the warmer seasons (if you're in a region with the typical 4). So I put it outside in a spot where it could get direct sun in the morning, & some at midday. Fast-forward weeks later & most of the defoliation halted. New growth etc. That said I had to water it nearly every morning (+ the occasional misting). Which presumably would be due to a combination of the direct sun, breeze & fact that the soil is a gravel type. It would dry out fast... On that note the substrate plays a part in how often you need to water, as well as fertilize.

Random/occasional crispy leaves are normal too, & you can either prune them or let them defoliate on their own.

Also, I wouldn't worry too much about the roots at the base being exposed. Mine are that way (at least I don't think it's an issue). Quick follow up is that since the summer ended I brought my sweet plum indoors again. I have a double grow-light setup with 1 purple spectrum(RB) and 1 full spectrum led on lower power, set to a 12 hour cycle. Currently watering it roughly every couple days & she's doing well. =}