1: no, the turn server is if everything else fails and a connection between the two players cannot be established
2: Debian has a turn server package now so it should be fairly straightforward to look into the package'S help on the Debian website
3: Yes (also consider turn server service providers so you don'T have to pay for a cheap VPS in multiple countries and end up paying a fortune every month)
4: You don'T have to pay for the bandwidth of your peer to peer games,
you don'T need a server unlike webcoskets, this means that with webRTC players can communicate directly with the game's host with low latency, and all of your game host game logic can be implemented in construct. If you were to use websockets as a relay between your players and host you have to pass through the server twice drastically increasing the latency in your game. If you don'T write your server code then good luck implementing server game logic.
webRTC allows for multiple data-channels both reliable and unreliable, whereas web-sockets are reliable only, it's easy to implement and there are multiple construct tutorials and documentation available and also users in the community with experience that can help you
turn server are cheaper than websockets server since all they do is relay data, they don,T have to run server code
note: nothing stops you from using both, such as using websockets for global chat or game invites or whatever and keeping webrtc for game room data