Hello Stoned Owl! I'm sorry to hear about your issues with the plants. Maybe I can help! I've been growing all things for many many years.
Some leaf curl is common. Almost all hybrids have leaves that curl, so do some heirlooms. Leaves also can curl if it is too hot, cold, dry, wet or there is a heavy fruit set .
Leaf curl is to be distinguished from leaf roll, a common condition seen early in the season when root and foliage ratios, root and ph problems, root bound, and root rot start to occur. As the plants mature sometimes this goes away. Sometimes it could also be pest that are sucking them dry and making the leaves roll.
This is what I would do! I would start with the growing medium, are you root bound? Containers or in ground? Not root bound, I would get some "Black Kow", it comes in a yellow bag at Lowe's or H.D., not too sure where you are located.
http://www.blackkow.com/
Throw several cups of it at the base of your plants. It seems to always fix any issues I have with nuts, normalizes things.. Black Kow is amazing!! You can actually cut a hole in the bag,cut water drainage holes in it, throw a plant in it and it will grow great!! Try it!!! Now water BlackKow and go to your plants. I would purchase a 3 in 1 spray for your plants. I use GardenSafe, it contains a fungicide, miticide, and insecticide. Good stuff! Spray them down and make sure you get the underside, very important.
Try this and hopefully it will help!! Here are some pics of plants. I have from many citrus trees, tomatoes to watermelon all being grown with BlackKow and garden soil. I only sprayed them once when they came up from seed to prevent any bugs or issues from arising.