Come on guys. "Things" have reasons in this game. It is completely not relevant if you personally find these "things" unrealistic, weird or not fitting in their world. These things are a part of ND's story and they will be explained however they want. If you see it in any other way, then I don't think you understand this game.
Tattoo: yes it could be the most blatent reason of hiding the bite, there can also be more to it.
Guitar: It can simply calm her, it can make her think of Joel, maybe she just felt like playing it, or maybe (blatently) she has a fetish for playing the guitar after killing people.
Also, maybe it was just there to let you know that in the new timeline setting she had learned to play from Joel. We are looking at a first trailer to set up the story / setting and this scene was made up 2 years ago. It does not necessarily mean that this scene is going to be in the game. Actually I think that during the panel Troy was trying to say that Neil told him that it was not in the game like this, but Neil was being a bit weird about it. (I didn't re-watch that part, this moment between Troy and Neil could have been about something else)
We are all just speculating which is cool and what they want us to do.