JJ, thanks for your input.
The fillings I removed recently were in all likelihood not the biggest problem though. They were very small and stable, and were removed carefully. The ones I removed 10-ish years ago were much bigger, and one of them was noticeably worn down from chewing. Also, I've had my symptoms, including POIS, for at least 20 years. So if I do have a mercury issue, the mercury is probably everywhere by now.
I respect your skepticism about the efficacy of Chlorella, but it definitely feels like something is happening in my limbs and skin in particular, which I don't know how to explain unless something is being absorbed into my bloodstream and is affecting more than just my gut. I've read somewhere about detoxing mercury that the most readily-available mercury is removed first, and it feels like something like that is happening in my legs: When I first started taking Chlorella, my calves and feet were numb and tingly to the point where I stumbled while walking and had to reduce the dose. Then gradually, the tingling and numbness receded downwards, and now I only feel it in my feet, especially the bottom couple of inches from the ground.
One thing to note, I guess, is that I do take small amounts of cilantro with my food. I think I read somewhere (sorry, don't have a source) that garlic and a couple of other foods can also mobilise mercury, and I have been eating a fair bit of that as it helps the gut and helps against mast cell activity. So maybe those things are mobilising the mercury and the Chlorella is capturing it somehow? I don't know. But the net effect, whatever it is, is definitely positive so far, and as the risk seems low-to-nonexistent, I'll keep taking Chlorella for now.
I'll definitely consider chelating, but I'm hesitant to go there right now because 1. all the info I've found about ACC online seems anecdotal and I can't find any solid sources with even rudimentary research, 2. I'm on a very limited budget, and importing DMSA and buying the other supplements I'll need is a bit much considering I don't even know for sure if mercury is my problem. (Presumably I'd need mineral supplements? Or does DMSA only chelate the bad stuff?)