I just wanted to offer you some hope, I guess.
I have an 11 year old Cavalier, called Jasper. He has MVD. He had the collapsing episode you have described, and spent the night at the vets. When we picked him up, they said he had less then seven days to live.
That was three months ago, and he is still going strong. My advice would be to listen to the complete rest bit, though. No walks, and no stairs. If Jaz is allowed upstairs, he gets worse for a while. We have a stairgate up now, and he is carried up and down. I thought it would be so cruel taking upstairs away, as he loves wondering the house, and loves being outside, but he's so much healthier he can play and enjoy his garden now. We are now hoping he'll make his birthday in April.
Good luck with this all...I have fibro so I know how CFS affects you, and how important pets are. I hope you've got a long time with him yet.