"The cellphone was broken by my younger brother."
Yes, one way to think of "坏" is as an intransitive verb, and another is to think of it as an adjective meaning "broken". Still another is as a "stative verb", as Chinese "adjectives" are often called when used as complements.
A person can't 坏 something, but only make it 坏. "弄坏" and "用坏" should both work.
From the English sentence, I would say '用坏' should not be the translation as '用坏' has more meaning i.e he breaks it "by using it" whereas '弄坏' would just mean that he makes it broken. '弄坏'should be the only accepted translation. If not there's also 摔坏, 丢坏, 敲坏, 咬坏etc. as possible translations.