BM problems... Need advice.

There's nothing wrong with magnesium oxide, IMO.  Very poorly absorbed, even in normal people.  It stays in the gut and therefore causes an osmotic pull for more water in the colon.  I like that you look into it though--smart :)  Never trust what we say without looking into it...lol

The low fat thing...no.  I don't think it will make you lose faster.  Fat just has a very low caloric impact on your body.  It's like a normal dieter eating celery.  Sure it has calories, but not very many for you. 

Ferrous sulfate?  BLAHHHHH.  No wonder you can't poop!  I would drop this to see if we can get you moving and we can add in a different kind of iron in a week or two.

 skim milk vs whole milk....  means you're drinking more sugar.  we absorb sugars a lot better than we do fat.  whole - or even half and half or cream is a better choice.