CrashD, you are absolutely correct that magnesium oxide is not easily absorbed, it's basically just a laxative. That's why I take a chelated form. Unfortunately many magnesium supplements on the market also contain a large proportion of un-chelated magnesium which doesn't help at all, or only marginally (as I discovered on one product I purchased).
I have attached this link which goes some way in explaining things, although one thing they say which is disputed by other sources, is that it can be absorbed through the skin (Epsom Salts). Who knows, I'm not a doctor.
http://afibbers.org/resources/magnesiumabsorption.pdf
Hope this helps and good luck. I did consider asking for an ablation at one time, but don't particularly like the thought of insides of important parts of my body being mucked about with!