What To Do About Shiloh

It's been a bit over one year since Shiloh was diagnosed with Myasthenia Gravis. Lots of ups and downs, and sadly, the downs have greatly outweighed the bright spots. In all my research, in all my vet's research, I cannot find a dog that has as many problems as Shiloh. It's probably because Shiloh has so many other things going on.
Way before the disease took hold, he was going deaf. He is now completely deaf; only vibrations bring a response. He has an interdigital cyst on his front paw that cannot be removed because of his disease. It bothers him a great deal. It doesn't hurt, but it is causing the toes to separate to accommodate the growth. I have to be vigilant and keep bitter lemon on his paw or a doggie bootie. He tries to chew off the growth. This just breaks my heart.
Then there are the anxiety attacks. Again, I can't find another dog that has a similar problem along with this disease. When they hit no drugs can calm him down and he cannot be crated. He just has to work through it.
My vet and I are in touch regularly - heck, I have SIX dogs, and two with freaky rare diseases. Lucky me. She asked how Shiloh was doing and in "normal" terms, he is doing just fine. He's able to eat and drink as he used to, he's able to elminate like a normal dog.
But he's not normal.
This was the #3 obedience sheltie in 2006, a Eukanuba National Obedience Invitational competitor. He ran World Team times in agility, was brilliant on sheep, and had a "just do it" attitude. All that has been taken away from him because of his health.
As we struggled to find a diagnosis during the onset of Myasthenia gravis, his rear end was beginning to fail. After the tests proved it was indeed MG and drug therapies began, his ability to swallow returned and most of the control of his rear end came back, but not completely. His left rear leg does what I call "the funky chicken" when he tries to run. Between the cyst on his front paw and his non-functioning rear leg, his favorite thing to do - run - is gone.
All my retired dogs still have jobs to do that help them feel important: Dusty's is to walk with me to the mailbox daily, Blitz's assignment is to help me with the laundry (with some obedience thrown in while folding clothes). Interestingly enough, Shiloh, who used to deplore snuggling of any kind, demands that he be the new Bed Dog. He does this as if his life depended on it, and will not leave me under any circumstances until I arise in the morning.
We have some private ball games as he still adores a good game of ball. I occasionally still let him have his beloved Teddy Bear - which I used to only bring out on dog show weekends - and his eyes always sparkle when the magical bear appears. He loves to talk to it and bite it so that it will squeak. He can do this for hours or until I take it away.
I don't know what to do. If I were Shiloh, would I want to continue like this? Is he living, or is he just existing? As mentioned there are occasional happy moments, but overall the sparkle has gone out of his eyes. My heart breaks for him on a daily basis.
While he's fully functioning in biological terms, I cannot tell of this is what he wants to continue doing.
Even though Blitz's condition is also deteriorating, it's clear he still wants to keep going. He's a happy dog and still living his life to the very fullest. Dusty at his advanced age is also a very happy boy. It's Shiloh. I just don't know what to do, or what is right for him.
His prescription for mestinon - the drug that keeps him alive - needs to be renewed in a few weeks. At that time, the vet will go over Shiloh thoroughly to evaluate his health and overall condition. I have asked her to give me her honest opinion on what she thinks will be the right thing to do.
In the mean time, I ask Shiloh to tell me, to show me.
If anyone out there has been through a similar experience, please share your insights.
I'm at a complete loss. And keep us in your thoughts. Thanks.