Testing RBC Mg seems reasonable.Might be pretty deficient, will ask my doc for rbc magnesium test.
Have you read about the normal phases of recovery from severe energy deficit? If Peat has written about it I have not seen it. What I've read has been here:
Patients and General Public
If you scroll down you'll see a series of articles about the Phases of Recovery. Part 5 describes the physical bloating and pain that usually accompanies the beginning of recovery from severe depletion. I think it's at least possible that this is part of what is going on for you. If that is the case, and you back off as soon as it starts to happen, then you may be stopping the healing process and resuming the damage. I think it would probably be worth reading the whole Phases series.
I would be very interested to know whether Peat has assisted people to successfully recover from anorexic and related conditions, and if so how, but I've not seen him talk about it so far.