Wow that sounds pretty bad. I wonder if any other ME/CFS patients have experienced that kind of muscle weakness. I know that at my worst, I found it impossible to do more than around 5 press-ups, do to the muscles feeling too weak. In those days, I was close to entering into severe ME/CFS territory. But I was always able to walk.
But I also feel as you do that the weakness is due to very low energy not strength. Because as the energy comes back the strength comes back pretty quickly. This has been my experience over a lot of years - I have gone in and out of this level of disability several times.