I think your tree has root rot,

The plastic is a weed mat. I wasn't watering regularly - but after I first planted it, it got watered daily for 20 minutes by my lawn sprinkler. I thought maybe that was too much water, so I turned off that zone. How often/much should I water it? Do these leaves look like they are being underwatered, or perhaps overwatered?

Maybe spider mites; maybe just underfed... you say nothing about fertilizer. BTW a sunny window indoors is roughly equivalent to full shade outdoors. Other than that, especially if you put it out... I would lop back the tops maybe 4-6 inches to encourage stronger growth. Oh, and enough with the copper; that is mostly for fungus; and I don't see evidence of that.

Wow you have run the full cycle with your plant. I don't have much advice other than check for more bugs and give it some time to adjust perhaps. I have a Chinese mandarin that I started from seed and I bring it inside in early fall before night temps get below 15 degrees or so. It lost quite a few leaves after bringing it inside this fall. I use mostly peat and about a quarter of garden soil, that has been working well. No bugs so far. I have been using rose fertilizer that has micro nutrients and flush every second time. I haven't used grow lights yet but will have to if I want fruit. best of luck with your plant.