A. A Critical Illness policy only covers a very limited set of impairments. And all of them are major impairments. With Disability Insurance it doesn't matter what the impairment is, just as long as you are unable to perform your job.
Also, some CI policies do cover "loss of independent living", but they require that loss to be permanent and not temporary. (which only covers a minor portion of DI claims)
Around 40% of DI claims are due to accidents and 60% are due to illnesses. A Critical Illness policy mainly just covers illnesses. There is the exception of paralysis, burns, blindness, and sometimes loss of limbs etc. But that is just a fraction of the possible types of injuries you could have from an accident.
DI is income replacement for any type of illness/accident that keeps you from working.
CI is just extra financial help in the face of a specific type of horrific accident/illness.