The figure I have seen is that muscle wasting drops to less than 0.2 kg per day once the body fully enter ketosis. Obviously, anyone considering a fast longer than a few days should do so under the supervision of a doctor who can monitor electrolyte levels etc. They can also check for the rare enzyme defect called Medium-chain acyl-CoA dehydrogenase deficiency, those with it are unable to fast safely.

Isn't 0.2 kg per day is a huge amount of muscle?? Thats half a pound a day!

I know this is true just anecdotally. I'm totally ripped after a 7 day fast with none of my muscle wasting away, just the fat.

for salt look up topics around:

https://medlineplus.gov/ency/article/000394.htm

Magnesium and potasium main but not only function is to regulate heart beat. Lack of them might lead to arrhythmia and other complications.

Not sure about muscle loss.