You state that you possibly have mercury and yet you are doing oral chelation. Did you do a heavy metal challenge test? Most people have heavy metal toxicity as a part of just living in today's modern world, so oral chelation can be one of those things that someone could do without really testing for it. It's important to note that moving toxic metals around the body can create reactivity by the body. If heavy metal toxicity wasn't a problem, then I'd probably address it later on once some other things were handled.
Both oils are good. Fish oils may be a more neutral choice than seed based oils. It seems that blood sugar problems are an issue with you that may also be effecting the thyroid function.