It is possible to see sarcoptic mange under a microscope, but yes sometimes it is difficult to find the actual mite to see it.
It is however very odd for a dog to have both kinds of mange at the same time (Demodectic and Sarcoptic), but this can happen. It would have always had the demodex and then been exposed to the sarcoptic somewhere along the way.
It is true that people can not get demodectic mange and it is also true that people can get Sarcoptic mange, which presents itself in people as scabies. You will just need to wash your hands very well after touching your pup and also wash all your pups bedding area really well....
The thing I find odd and not to get into questioning your vet is this:
( The vet gave my pup a shot of pen-aqueous 100ml and a couple tubes of advantage multi..he is also on an antibiotic )
The pen-aqueous 100 ml is basically a Penicillin injection, any antibiotic by mouth in a pill form or a liquid is just that an antibiotic and probably all used to treat a secondary skin infection from the mites either sores or from scratching. The Advantage Multi treat for heartworms, hookworms, roundworms and whipworms.
There is nothing in any of these items that kills demodectic or sarcoptic mange......So if this is all you got or all they did I would make sure to call them and ask what they are going to give you to kill the mange...If they are not familiar with treating it then it may be wise to seek out another vet who is........Otherwise, your pup will be getting worse and not better.......