hey there LP
personally i think when you're low in iron, you should work to correct it, but should not do so indiscriminately without considering some other variables. in ms there is a large camp concerned with iron levels but it's a complex subject. just from what you wrote i think a few things are coming into play.
first, about interactions between iron and magnesium:
on the face of it and only from my perspective, you could be taking in iron to the detriment of your magnesium absorption resulting in nightly leg cramps.
second, about interactions between iron and zinc:
if you are consuming tons of iron and it's just not showing up in your blood, are you excreting it, or is it being mishandled in some way?
i suspect, and i'm no professional by any stretch, that as an MS patient your zinc levels are down. (and by the way if you are low in zinc, it may have affected your uric acid and D3 status among other things also typically found to be low in MS patients).
iron competes with zinc for absorption. so i would say that with a high iron diet, you could be driving your zinc status down even further (taking your uric acid and D3 status along for the ride) - and i don't think that would be any good for an MS patient.
supplementing zinc and iron is complex. let's say you're anemic - if you supplement ONLY iron you can effectively improve your anemia, at the expense of your zinc status. likewise by supplementing zinc to correct deficiency, you can drive your iron levels down. folks that are low in both nutrients and who then supplement both nutrients see increases in both. so it's a balancing act. here's the abstract: http://www.ajcn.org/cgi/content/abstract/49/1/145
with the above in mind, some things to consider:
liver has both iron and zinc to offer. if you eat it by itself, your status in both nutrients should improve.
if you eat oranges with that liver in the same meal, that will increase the iron absorption, possibly to the detriment of zinc absorption.
if you now add lentils to the plate in that same meal, the high level of phytates present in lentils can bind up the zinc, and your zinc absorption decreases further (same thing happens with cereals and other high phytate foods.. and magnesium is also driven down by phytates - here come the leg cramps).
so starting again, if we put the liver on the plate it has a balance of zinc and iron.
if we add oranges to the plate we boost the C and help the iron.
now instead of adding lentils let's add crimini mushrooms instead, you add no vitamin C to the mix, and 1mg of zinc, plus some beneficial potassium and selenium..
then add a cup of boiled spinach or swiss chard (phytates in greens can be reduced by boiling), and you get a nice mix of high magnesium, zinc and iron.
the balanced intake of zinc and iron with lower impact from phytates should improve your iron status in step with zinc status, and then your uric acid and D3 and whatever else can benefit also.
there's no reason to completely avoid high phytates in foods.. all you have to do is be careful and balance it out over time, and figure out which ways there are to mitigate phytate levels in foods (such as choosing sprouted grain breads etc)
it's all just conjecture from a non-pro, but that's what i got for ya.
by the way, cacao is rich not only in iron, but in magnesium and zinc as well
a nice square of 70% cacao dark chocolate should do you no harm