Such a difficult one to call. Personally, I don't think there is anything in Law that says a player can't go down under a challenge, nor is there anything in Law preventing a player from appealing for a free kick/penalty. For me the big thing is 'contact/appeal'. A player goes down without contact and appeals - it is simulation. Regardless of how much rolling around he does.
Feigning injury is is even harder to call - we are not medics so we can;t tell if someone is genuinely injured or not. Players who 'require treatment' are now instructed to leave the FOP so that a fake injury doesn't hold up the game. If it's a genuine injury, then the team that has been fouled and received the free kick, are now disatvantaged by being reduced to ten, and there's a 10% chance it's their top free kick taker who has been ordered off the FOP!
Likewise, I don't see anything in the LOTG that prevents a player from manipulating the situation in order to ensure contact is made with an opponent and claiming that as a foul - as per the Young example. Clearly, it was not a foul tackle but Young didn't simulate anything either, there was contact, he went down and appealed. "No foul, get up and get on with it and stop pansying around son". The fact Young stuck his leg out and dragged the foot doesn't really come into it, because again, players are allowed to stick their legs out in the course of a game.
Similar thing happened at Chelsea against WBrom - Chelsea attacker clearly drew the contact in on himself, and went down. Again, there's nothing wrong in positioning yourself between the ball and defender as in this case, and nothing wrong in going down under the challenge/contact and asking the Referee to make a decision. If it wasn't given, he'd have given up possession in the dying seconds of a game they were losing, instead he took a gamble and saved his team a point.
So for me, simulation is just that - pretending something happened when it didn't.
I don't have a problem with players going down and appealing, if they don't accept my decision then they are heading for a dissent card.
Feigning injury is the one I always struggle with......