Maybe the xmr price is stable because some whale(s) sell the right amount at the right time, balancing supply to the present demand.

BTC wasn't stable when it was "the only coin used as an actual currency" way back in time

Is it possible to, without any extra knowledge, send a transaction back to whoever sent you monero?

In say, bitcoin, I know the address of whoever sent me btc, so I can send something back to them without them providing us their address "explicitly". Can something similar be done in the monero protocol?