No, I didn't think of it that way, just simply if you want to participate in the hypothetical POS Network, you need Bitcoin and can ONLY use Bitcoin to receive the POS token.
That's the plan: to have a network, where you can receive coins, only if you sign your bitcoins. Of course someone else could sign its own coins on your behalf, there is no way to stop that.
You can buy if you want to, but the context still is, all Bitcoins in existence are created into existence as block rewards received from mining.
Yes, but the current difficulty means that a lot of energy has to be used. It is possible to convince people to turn off their miners gradually (by convincing them that they can earn more by making signatures and staking their bitcoins), so the difficulty would gradually reach the minimal value. Then, it is possible to protect the whole network mainly by Proof of Stake, and leave a little protection of the Proof of Work, just to make it backward compatible.