I just went to copy the PM I sent you yesterday, and can't find it, so I guess that I'll try to remember what I said and start again, lol (no wonder you didn't get it, if I didn't send it properly!).
I had chronic diarrhea for about 2 years, and tried altering my diet in many ways, including adding fiber and lessening how much of it I ate and by eliminating other foods like dairy. I also tried 4 different (and very expensive!) probiotics and nothing helped at all.
Before LIS, I used to sit at my computer every morning and check my emails and forums and would always have a cup and a half of coffee. After the surgery though, because I couldn't sit for the first couple of weeks, I decided to have tea instead, because lying on the couch with coffee just seemed "wrong". I did not have one teaspoon of diarrhea after the surgery, and the only conclusion I could draw was that it was a result of this change. I then read up on tea and discovered that it is an astringent that could help cure diarrhea. I had also read that coffee could
cause diarrhea, but because I'd been drinking it my entire adult life (I'm mid-50's now) and only had a cup and a half a day, I thought that that couldn't
possibly be the problem. Looks like I was wrong.
Another possibility, raised by my CRS, was bile acid diarrhea, and he said that he'd help me investigate that after the surgery, and that could easily have been treated by a drug (Questran) that was used for high cholesterol before the advent of statins like Lipitor. That clearly wasn't my issue, but perhaps it's something that you could look into.
I definitely wouldn't be taking Magnesium though, if you already have stools that are too soft, because that could make the problem worse.
I hope that things improve for you very soon