Sabrina Hahn has a book on looking after citrus. I haven't read it, so I don't know what the problem is, but I know the book exists.
As someone who eats mandarins I'd say burn it down and start again.
Not sure yet but have you tried BigGardening?
Citrus trees are frequent targets of various aphids and mites, I'm no expert but have a lemon tree which suffers from sooty mold which is caused by pests like scale, aphids, mealbugs etc. I've been treating it with a oil which is sprayed on the affected leaves and branches. You can buy various brands at Bunnings for $15-$25 for a 3 liter spray pack.