I wish I could offer advice but I have not been faced with a puppy with that bad of behavior problems. I am glad for the long post Carol(member name Archie) wrote to you about her experience with Buddy. At least you can hear from someone else that has been through something similar with their puppy.
I can offer some suggestions but I do not begin to think I know what it is like to be in your situation so feel free to toss my ideas. The only idea I got from reading through your post was that you said you had tried so many different approaches to training your Rocky, yet he is only 5 months old -- so I imagine you have had him for only a couple of months. Consistency is a BIG part of successful training, especially with our willful little toy breed. Could it be that you are changing your training methods too soon and too often? Just an idea to think about, not saying anything for sure.
I imagine most of the misbehaving is because of not being socialized as a puppy, perhaps being taken away from the mother too soon, and poor breeding practices. Some may be due to him sensing your health problems as another poster suggested. I find mine do sense when I am physically or emotionally hurting -- but they tend to lay on me and be quieter -- extra loving not wilder.
I can't imagine going through all you are and also having to deal with a puppy that has this bad of behavior problems. It might be that you should not continue to struggle with this at this point in your life. I do agree with the others though
PLEASE, PLEASE do not turn him in to a shelter/pound. If you tell that he has bitten, many states have a mandatory kill policy. Much better to relinguish to a rescue letting them know just what you have us in your post. They will find someone that has the time, energy and hopefully the experience to deal with extreme behavior problems. At least the little guy will get to live.
Even at the best of times, a yorkie puppy requires a lot of attention and patience to get through the puppy stage. Sometimes I think my Rocky loses his mind for an hour or so a day. But luckily he has always been well socialized (I raised him!
) and wants his loving enough to turn around and act right just as I am wondering what in the world to do. With your situation it might be that a grown yorkie would be a better match. Some puppies are just a lot more difficult than others, regardless of their origins. But I imagine a puppymill makes it a lot more likely behavior problems will abound. With everything you have to deal with on a daily baiss, I would not hesitate to change the situation if you feel you have given all you can to trying with your Rocky. It might just be in both of your best interest to rehome. You could try here on YT. There may be someone willing to work through his problems.
I would not let one mismatch turn you away from yorkies though. I think they would be the perfect breed if you had a little older, more settled one. I have dealt with a lot this past couple of years and cannot imagine not having mine. They are such a huge comfort and source of enjoyment. Saying a prayer for you, Rocky and your whole situation!