I wouldn't deprive mine of avocado, apples and mushrooms. They enjoy eating these foods. I'm sure they could survive if they ate enough turtle food, but you are also getting moisture in them with fresh foods. And if the turtle food was too high in protein or something, you could cause other problems. So for a vacation or something, sure. But I wouldn't rely on it.
I bake the leaves at 200 degrees for about 2 hours. I wrap them in aluminum foil for safety. As far as changing the substrate; I am pretty new at this. But my plan is to change it every 6-12 months. I think it will depend on what the substrate looks like and how much wood is left in it. I have changed some earlier than 6 months if things don't look right. I think I just have to play it by ear!
Mine feed on decaying hardwood leaves, the rotten wood that is their substrate, fruits, vegetables, the occasional dry anchovy, and mushrooms resulting from my beetle rearing. I've also recently started providing mine with Manduca sexta pupae that were destined for the freezer at work. They seem to like their new source of protein much more than vegetables and only a little bit more than fruit. I just cut them open and the millipedes eat the contents.
Mites are inevitable and part of the environment. They're also a common hitchhiker in grain-based foods. You'll just have to deal with them when they occur and avoid letting them get out of hand. I recently had some casualties because I wanted to avoid killing babies in the substrate and the swarming of the mites stressed out the millipedes until I lost three adults. I ended up pouring boiling water into the substrate after getting as many babies out as I could find and then treating the adults for the swarms of mites crawling on them. I did this by setting them in low humidity for a few hours and then using corn starch to dislodge the mites. The fine particles of dry corn starch made it so the mites could not maintain a hold on the millipedes and could be blown off and rinsed off. As far as I could tell, I had four different kinds of mites infesting their enclosure in addition to the commensal mites on the millipedes. They're now mite-free and are behaving more normally than when I first started observing mites in their enclosure--no more curling up for days on end and ignoring food, excessive grooming of their legs and undersides, and no more erratic and jerky movements.
I try to avoid leafy greens and most fruits unless I grow them myself. There's just too much potential for residual pesticides since it's the only way to avoid getting pests on huge fields of these crops. Systemic and translaminar pesticides are commonly used in agriculture and cannot be washed off since they are absorbed by the plant.
I have some kale in the garden grown just for my pets and I freeze fruits from my fruit trees for use throughout the year.