I'll agree with the captions, but none of the three examples were crippled by mercury poisoning.

"Symptoms typically include sensory impairment (vision, hearing, speech), disturbed sensation and a lack of coordination. The type and degree of symptoms exhibited depend upon the individual toxin, the dose, and the method and duration of exposure."

"Common symptoms of mercury poisoning include peripheral neuropathy (presenting as paresthesia or itching, burning or pain), skin discoloration (pink cheeks, fingertips and toes), swelling, and desquamation (shedding or peeling of skin)."

given the bags under Korra's eyes, she is likely in some kind of constant burning pain in her legs and arms. Toph and Ming-hua were likely not in pain from being blind and born without arms (not sure if Teo had pain or not, since the flood was what paralyzed him from the waist down). yes, Toph found a way to blind-earthbend and earthbend-see, and Ming-hua developed water limbs to an amazing level, but they didn't have to endure pain while trying.

Yeah, because they didn't have poison weaken their entire system.