I feel so sorry for your guinea pig! Please do try to stay calm for your piggy.

Remember many times the internet can be a helpful thing, but sometimes it can mislead you with incorrect information. We run into that day in and day out at work. We once had a client that was convinced, after looking on the web, that her dog had a horrible tumor because she'd seen a pix online that looked just like the lump her dog had. She had printed out PAGES of stuff from this site and brought it to us. She'd gotten herself worked up into a horrible panic convinced the dog was going to die any minute of cancer or would require chemo or surgery immediately. After the vet examined the dog the 'tumor' turned out to be a simple oil filled cyst, they lanced it, squeezed out all the pus, gave the dog antibiotics and some salve and the dog was perfectly fine and still is to this day tumor free.

My guinea pigs never had any skin issues and where I work we don't see guinea pigs at all so I have no personal experience to give you in those aspects. However relating this to dogs/cats in my experience at work the dandruf you mentioned could/might be chewing lice. We've seen a few cases of that at work before, the treatement for that is different than for mange mites. Also severe dry skin can cause dandruf and severe itching which is a different treatment also. There are also three types of mange mites Demodectic (sp?) and Sarcoptic (sp?) and Notoedric (sp?) I believe they can only tell the difference with a skin scraping.

As for ivermectin in guinea pigs I'd be very cautious about that without a vet's approval. Guinea pigs are such fragile critters and things that may be ok for a dog or cat can be fatal to a guinea pig or require substantially smaller doses. I also realize guinea pigs need faster treatment for things being they are smaller animals. Is there any type of emergency vet nearby you can at least call for advice? Or do your area vets have an after hours number to call for assistance? Or even call a vet's office outside your area just in case they have an after hours phone number. It's worth a try before you go ahead and do anything yourself.

Good luck.