Regarding calcium and magnesium supplementation, you may just have to give it a go and see how you feel. It is an individual thing.
RBC magnesium is a fair indicator of magnesium status, a place to start but calcium status is very difficult to gauge. If it is really off it will show up in lowish or highish ionised calcium in blood, not necessarily outside the normal range, just at either end. Outside the normal range indicates a serious problem since the body goes to extreme lengths to regulate blood calcium.
With the benefit of hindsight I realise that my own marginal calcium status was reflected in a high 1,25 diOH vit D to 25 OH vit D ratio - ionised calcium was always dead on mid-range. (1,25 diOH D, the active form, rises when calcium is in short supply, signalling to increase calcium absorption from the gut.) As soon as I added calcium, the D ratio normalised and my temporary intolerance of oral magnesium disappeared.
Start with magnesium, particularly if you suffer from glutamate excitotoxicity. Magnesium antagonises the NMDA receptor.
You may need to experiment with type and with route. I once had a lot of trouble tolerating magnesium but eventually found the bisglycinate form suited me (usually called chelate). It's impossible to predict - you just have to try.
You mention diarrhoea - you may need to build up bowel tolerance, so start low - maybe 100 mg.
The topical route is excellent and I find it very helpful for muscle pain. I use both magnesium oil (chloride form) and a concentrated Epsom salt solution (sulfate form), maybe 400 mg magnesium total. The magnesium solutions are quite irritating/ drying on the skin, especially in the large amounts that I use so you'd need to follow with oil/body lotion. (You can buy ready made lotions with magnesium but the ones I looked at were not very concentrated and I didn't like some of the ingredients).
Sometimes intolerance of magnesium means that something else is limiting - maybe B6, or a trace mineral.
Once you have a moderate amount of magnesium on board, you could maybe try some calcium.
Your quotes do typify the demonization of calcium which has made us fearful of the supplement, but it is all about balance.
No time to look at the stool test now, will do it tomorrow.