You said, "When I re-potted it last month, though I didn't check closely, the root mass look typically like a pot-bound plant. I didn't notice any mushiness or rot."
It's always best to check the rootball and loosen the outer roots if the roots are circling tightly.* There are plants with delicate roots that can't tolerate the root disturbance.* If a plant is severely rootbound and you overwater, the water can stay in the tight mass longer then is good for the plant.* The other side to that is the tightly wound mass can't take up enough water.
You said, "Is it possible that over-watering and/or over-fertilizing would show the same symptoms?"
With root rot you will usually see browning of the stems.* With fertilizer burn the leaves closest to the base of the plant and the older leaves will usually burn first.
I think I would unpot the plant and take a close look at the roots.* If it's severely rootbound I'd try and do as they did in the first link above or even wash off all the soil.* Cut away any brown or broken roots and any brown stems.* You might lose the plant, but at this point you may be able to save it.* I would wash the pot in solution of one part bleach to nine parts water (wear old clothes as bleach will splash), rinse with a 50/50 mix of vinegar and water, rinse again with plain water and let it dry.* Repot with fresh soil so you are certain it's not infected with any fungal spores.
I'm not a fan of sulfur based fungicides with edibles as some people are allergic to sulphur.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.