The addition of -ity to an adjective results in a shift of stress to the antepenultimate syllable; that is, words in -ity are stressed on the last syllable before the -ity, even in cases where this syllable is part of another suffix (as in words in -ability and -icity).
adjective, dumber, dumbest. 1. lacking intelligence or good judgment; stupid; dull-witted. 2. lacking the power of speech (offensive when applied to humans): a dumb animal.
2. The distinction you're making between 'dumb' and 'stupid' is a nuanced one. Your answer would therefore benefit from a further explication of what you see as the difference, and from some supporting evidence (as it stands, it's just an assertion).