Hate to say it but I dont think its his soil, I didnt read every single post, but it sounds like youve tried changing soil and such and still have the problem...now tiny brown spots, I saw dirt on your leaves in the first pic....and they are yellowing to a yellowy green and drooping right? sounds like spider mites to me they suck the life right out of your plant especially with an untreated infestation.... check under the leaves with a microscope (or you may be able to see them with the naked eye, i can) but on the underside of the leaves there may be tiny brown/red/white bugs and look for TINY little webs anywhere around and on the plant, if you find either of these its surely mites... Im pretty new too but I think thats what you got goin on ... let me know either way
(IF IT IS MITES) go to your local hydroshop they will have pure neem oil, the should also have safers insecticidal soap (safe for plants and animals) mix about 8ml of pure neem oil with a liter of war-hot water...then add safers soap as needed, shake and mix until MOST of the yellowy soap-neem has mixed, then immediately transfer to a srpay bottle and drench your plants (espeically the undersides) with the concoction once every three days, and spray with normal water on the leaves and underneath every day this will keep their life cycle inhibited....you will never rid your self of them, you can only control them....good luck and i hope this helps
(if its not them im sorry =((( )