2. "I have been playing the piano since nine o'clock."-- The usual, accepted way.
1. "I have been playing the piano from nine o'clock."-- Another way which some will not accept. My grammar book says; "since" for perfective tense, do not use "from". But is this really so?-- No, not really.
3. "I have known her from my childhood." -- Another way.
How would you guess how the "some" people would feel when they see this kind of sentence?-- They would feel it should be 'since'.
How much percentage would you think the "some" people account in native English speakers?-- No idea. It is a grammar point, though, and few native speakers know any grammar points.