Nigel, Your idea of fumes from an exploding missile has merit in terms of white hair, orange powder, orange/brown skin, slits in the tent and so forth. But if the last photos do show a missile exploding, why would the hikers walk to the trees? Wouldn't they have felt safer back inside the tent? Yes, the fumes from nitric acid would have killed them in or out of the tent, but they didn't recognize the danger of the situation. And I don't agree with your idea of a military exercise. There's a complete lack of evidence of such an exercise. Wouldn't many Soviet troops have died from the same toxic missile fumes?
I'm always open to new explanations for this tragic event, but a naturally occurring phenomenon is the simplest, most reasonable and I think most likely cause.