Stuart, since you are very underweight and have plenty of muscle ache which might be due to a magnesium deficiency, imo it would be a good idea for you to eat plenty of raw sunflower seeds.They are high in magnesium, and also have other minerals, protein, and plenty of calories as well as fiber. I get them unshelled for under $2 a pound. Other nuts and seeds also have magnesium, but not as much.

"Also i've started experiencing lockjaw. Its very hard for me to open my mouth very wide."

It sounds like you may be grinding your teeth at night. Do you use a mouthguard when you sleep? I had one made by my dentist which was expensive, however there are ones available in the drugstore for around $25.

It sounds like you probably do have a problem with mercury, but also probably have some vitamin and mineral deficiencies. You should work out a supplement regimen. Are you eating enough fiber? Try to drink more water. Focus on getting enough minerals, especially magnesium and zinc. Vitamin c and b vitamins are very important. You could start with 1,000 mg vitamin c twice a day, and a half a B50 tablet twice a day. taking a vitamin D supplement might help greatly with the muscle ache. I suggest 2,000 units a day. You could also try getting more sunlight. Do you get enough calcium?
Plain yogurt is a good source of calcium.

Have you done a parasite cleanse? Many here have had parasites.
Many of your symptoms might be caused or at least worsened by parasites. There are parasite clease formulas in vitamin stores. Some contain many herbs. The better ones seem to contain at least these three herbs, wormwood, black walnut green hull extract, and cloves. Most good cleanses last around a month. Humaworm, Kroeger Wormwood Combination and Paragone are very popular. Wormwood Combination is probably the least expensive at around $5 a bottle on the net(2 bottles are needed).