Ginny got the temporary ability to speak Parseltongue when Riddle possesed her but was not the heir of Slitherin because she could not control the Basilisk herself. Lucius gave the diary to Ginny because he knew it would be dangerous and gave it to her in the hope that it would hurt the Weasleys. I think that only the word open in parseltongue would open the chamber because Tom would have needed to shut it again when he was at school so he probably said something like close so it would shut again. I think he meant that nobody would have known where her skeleton was so it would have stayed there. At the begining she trusted hin but as she began to realise the truth she began to fear the diary. When Harry destroyed the diary the part of Voldemort's soul that was in there was killed so all the power it held in this world was destroyed. Salazar original purpose for the chamber was never mentioned, maybe it was where he liked to go down to and think, or maybe it was the original Slitherin common room, we may never know.