My big girl goes blue for 2-3 weeks before she actually sheds. As for the mites I would switch away from your supplier especially if its a reoccurring issue. Just because the mites are wood mites doesn't mean that they can't effect the health of the animal. Having mites in the enclosure is very stressful for an animal and could explain both the fasting and the prolonged shed.. I would treat the enclosure with PAM and switch him to a new substrate.
Hope this helps and good luck,
Chris.