Basically, the presence of heavy metals can screw up a body's "air conditioner." So the person may feel really hot when other people feel warm and really cold when other people feel cool. I believe it's because the body takes so much energy to cope with the effects of the metals, it doesn't have the energy necessary to keep the body warm/cool. I'm not 100% sure about that though.
Be skeptical about any metals test you have done. They are infamous for giving false negatives. Some are better than others, but there are many people who have negative results from the tests and still feel better after chelating.
You asked the million dollar question. Why don't they stop using Hg? The reason they use them in fillings is because silver by itself can't be easily used as a filling. They used to heat the metal until it melted before putting the scalding hot metal into people's teeth. Adding mercury made it flexible enough to use more easily. Its use started a long time ago before the toxic nature of mercury was fully understood. This was a time when, for example, hat makers were using it to make hats and becoming "mad hatters" as a result of mercury toxicity.
The American Dental Association was formed more than a hundred years ago to defend the use of silver fillings. The dental industry has continued to defend their use even after the toxic nature of Hg was determined. The industry's arguments have varied over time. The first argument was that mercury doesn't leak from the fillings because it forms a tight chemical bond with the other metals in the filling. This was shown to be untrue, so the industry changed its argument to say that not enough mercury leaks from the fillings each day to hurt people. This is correct, in my opinion. One day of leaking is not enough to hurt most people. The problem is that the leaking goes on every day of a person's life for as long as they have the fillings. The accumulation of mercury eventually makes a person sick.
I think the dental industry continues to use the fillings because they are cheap and admitting they are dangerous would open the industry up to many, many lawsuits. Perhaps, they genuinely believe that putting this toxic substance into people's bodies for many years is safe despite the fact that the government requires Amalgam fillings to be classified as toxic waste when it's outside of the body.
The poor nature of heavy metals tests has added to the problem. Doctors often dismiss the idea of metals being a widespread problem and support their opinion with the tests which so often have false negative results. How can we trust studies which, for example, dismiss the idea that there's no connection between heavy metals and autism when they are using those same questionable tests to determine metals level?
Sorry this post has gone on for so long. I have to go to work now.