Answer: Although long, your question was detailed enough for us to get the pertinent background information on your Pachira. It's too bad that you didn't transplant your Pachira as soon as you recognized the pot had no drainage holes because we think the problems your plant is experiencing come from root rot - lack of proper drainage. I know you've been careful with your watering, but stagnant water was probably collecting in the bottom of the pot. As the roots grew into this slurry of stagnant water and potting soil, they didn't get enough oxygen and root rot developed. I think if you unpot the plant, you'll find the roots will be mushy and there will probably be an odor. You can try to save your plant by repotting into a more suitable container and using fresh potting soil - but there are no guarantees. It may be too far gone to save. I think it's worth the effort, though.
Best wishes with your plant!
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