1. I think that who ever the Headmaster is at the time is the master of the elves.
They as elves show deference and respect to all wizards and witches but I think that they consider the Headmaster to be their master.
When headmaster's change then the elves have a new master.
Like when Sirius' mother died Sirius became Kretcher's master.
2. Oh yes absolutly. And Hogwarts has Many secrets as DD himself once stated.
"I would never presume to know all of the secrets of this castle"
but I'll bet the house elves know them all.
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