Since this was the first question, I’ll start with this one.
As far as I’m aware, in the more “advanced” holometabolous insects (basically, all the insects with complete metamorphosis other than various Neuropteroids) nearly everything is “recycled” during the pupal stage, but I think a fair bit of the nervous system is retained. Yes, it undergoes some drastic changes in organization (especially the fusion of larval ganglia), but I believe a fair number of the cells - especially those in the head - and their connections to one another survive the transformation. It would not surprise me to find that the degree to which the nervous system is broken down varies from group to group, but I doubt that anything has 100% broken down.
The next question was about chitin, and yes, chitin is a polysaccharide like cellulose, and I wrote my original response hastily and didn’t catch my own mistake until later. There is a fair bit of protein in the matrix that makes up the exoskeleton in addition to the chitin; that is, the exoskeleton is a lot more than just chitin. It has all sorts of different layers with different properties and composition (e.g., an outer wax layer), but that topic is pretty tangential here.
The next question was whether the imaginal discs had the same DNA. Yes, every cell in the body has the same DNA, discs included. It’s simply that the discs express a different set of genes than are expressed by the other cells - they’re not like stem cells (i.e., they are not undifferentiated), it’s just that their “program” is a different one. Some of the most fascinating research ever done in cellular/tissue biology has dealt with insect imaginal discs, especially the way they repair themselves if damaged.
The final question (also tangential) about parasites is simple enough: lots of insects make their livings by feeding off of something else - if this feeding is non-lethal, it’s called a parasite, and if it’s lethal, it’s called a parasitoid. Nearly every species of non-marine arthropod has at LEAST one parasitoid that attacks it. That holds true for essentially every insect species that has a caterpillar stage, as caterpillars are generally fairly vulnerable. Your average butterfly or moth caterpillar is potential host for at least 5 or 6 different parasitoids, most of them various types of wasp, but also various flies. Moreover, those parasitoids are THEMSELVES targets of other parasitoids, known as “hyperparasitoids”. So, if you were to spend a few years raising tomato hornworms in your garden, for example, and raised a few hundred of them and kept everything that came out, you’d eventually compile a collection of something like a dozen or more different species of insects. If you also looked at the eggs and pupae, then you’d find still more parasitoids that only attacked those life stages (parasitoids that attack adult insects are fairly rare, though some do exist, like conopid and pyrgotid flies). There are lots of professional entomologists who study this sort of thing, and it’s a truly fundamental part of insect ecology. Some of the life histories involved are truly mind-boggling. Look up the wasp family “Trigonalidae” some time for one such example.