Player versus Player is mostly about defeating other human controlled characters in cities where it is allowed. Since human characters have a much better control over their Gears and update their Charm collection at a fast pace, the player you defeated yesterday may completely slaughter you today if you don't adjust your Gears over time, too. That's why PvP is much more focused on competition between players.

Yes. We use the Elo rating system. While perhaps not being the best, the Elo rating system is widely used in games which involve PvP. In addition, we had to put some limitations such as you cannot gain rating if the level gap is too huge, if you've already fought the opponent in the same day etc. We are trying our best to make the rating system fair to everyone, but high level players are likely to appear on top of the PvP list most of the time thanks to their experience and knowledge about Estiah.

We do allow anyone to fight anyone in PvP, but if a high level player is repeatedly killing a low level player, he's probably wasting his time and his precious action points. He will not gain any rating or item, it may be a harassment to the low level player, but by doing this he's slowing down his character's own progress. The low level player will eventually catch up and take revenge.

Yes. There is nothing to stop you from doing so, but you'll probably regret it later. You'll miss lots of interesting Charms and Items, and of course, lots of fun too. Even though PvP has its own rewards, PvE rewards appear to have a slightly better quality, because we like players who enjoy our game to its utmost :)

No, as long as you travel between unsafe cities, you will likely be attacked by other players. There is no way to take you out of PvP system, and there will not be, probably.

Don't worry though, you'll lose near to nothing if you are assaulted by other players. Your PvP rating will go down but since you don't care about PvP, you don't look at your rating, right?