Phil,
Like you I have IBS, but, unlike you, it's predominantly diarrhea. Like you, I have been ZC for a while (>6 months, in my case). Unlike you, I eat cooked meat. Like you, I have been plagued with muscle cramps. I get them at night in my legs and in the day in my toes. Like you, I experimented with electrolytes.
I had read a few places that I could increase my intracellular magnesium, without getting diarrhea, by bathing in epsom saltwater. This study
http://74.125.47.132/search?q=cache:KO8yMO7uJtAJ:www.epsomsaltcouncil.org/articles/Report_on_Absorption_of_magnesium_sulfate.pdf+absorb+magnesium+through+skin&hl=en&gl=us didn't measure intracellular magnesium, but it is suggestive. Bathing in epsom salts purportedly has the additional benefit of improving one's skin.
So, I decided to replicate the study I linked to and take an epsom salt bath nightly for a week. After about 4 days I got very, very bad diarrhea. I strongly suspect that the diarrhea was caused by the baths. I stopped the baths and the bad diarrhea ended as well. At some point I will need to test this further by taking the baths nightly again to see if I can replicate the bad diarrhea. But I am not ready for that yet.
In any case, I just wanted you to know that a 12-minute bath in warm or hot water, with 2 cups of epsom salt added can (1) increase your intracellular magnesium and (2) provide you some relief for constipation and (3) help with your acne, if it's on your body and (4) do all the other things that oral magnesium does with regard to muscle relaxation and the cardiovascular system.
I am new to this forum, and I am not a paleo zealot. But I suppose one could make the case that bathing in epsom salt is something that a paleo person could have done if she was fortunate enough to have lived in one of the few spots where there are high concentrations of the stuff. But she could not have taken magnesium as an oral supplement (unless there is some high-magnesium substance that exists naturally).
What drew me to epsom salt bathing was that I could get the magnesium while bypassing my digestive tract. Unfortunately, it didn't work for me. It's something you might consider, however.