Interesting questions being raised here. I too have often wondered whose responsible in such cases. I prefer aisle seating, but am often very concerned when passengers are putting oversized/bulky/heavy bags into the overhead above me. There have been a number of times where someone 'loses the struggle' of getting their bags up and it ends up falling down on the aisle armrest or someones lap. Usually, the response is a mumbled "sorry" and a reattempt to get it into the overhead bin.
As for departures, does the fact that the airlines make the statement "be careful about opening bins as contents may have shifted...." does that absolve them of any liability, as they have 'warned' you and thus would it now be 'your' responsibility to take care????