Can it be good for the crypto economy if a load of money suddenly appears out of nowhere?
It didn't appear out of nowhere. It appeared because people are willing to pay that much for the coins. Of course for every buyer there has to be a seller.
It just means that right now, the market believes that BCH is worth about 0.12 BTC.
How much BCC immediately got converted back into BTC?
It's hard to tell, but when the split happened, BCH's volume with BTC was about 100-200 million dollars.
What's to stop people forking again, just to make a killing like this?
Anyone can fork at any time. The forked chain is only worth something if the market perceives it to be worth something.