If it was my plant, I would remove it from the soil, wash off all the roots, swish it around that soap/water mix and repot it in a fresh soil mix. Remember to scrub the pot, too!
I really don't know if that's the proper thing to do, but it sounds like you did everything you could to the top of the plant and that didn't work. Logic says the culprits are in the pot of soil.
I think the gnat-like bugs are due to too much moisture.