You'd probably not find too many people to argue with the assertion that Man Utd haven't been great all this season but until two games ago they could still have set a record points total for the EPL, by most measures that's pretty consistent.
I've only seen replays but it looked like he stayed down for quite a while and was grinning while facing away from the referee. Cheating might be debatable but it certainly didn't look like he was injured so to stay on the ground until another player was sent off is a touch dubious.
On the first point, no, consistency in winning isn't consistency of football, you've had a few games you've played fantastic football, interspersed with games they played diabolically awful football and games they've just played half decently. They haven't played consistently good football throughout the year, I wouldn't even say on average they've put in good performances.
As for points totals, points totals mean nothing unless Utd were playing the same teams, with the same players, the same managers and same point in time as previous points totals. Getting 80 points this year doesn't mean a team has been better than a team that only got 60 points a decade ago. The league is woeful this year, the overall quality up and down the league is an utter joke compared to the league Chelsea got their record in, or Utd got their previous highest points totals. The fact that a team as poor overall as Utd was on course for the highest points total is merely a reflection of how poor the league has become, not a reflection of how good Utd are.
With luiz, he wasn't hiding his smile, the linesman could see, has no one even been hit by a friend and laughed instead of reacted, sometimes life is funny and you laugh. He made no attempt to hide it, he quite obviously didn't pretend he was in more pain than he was, he got kicked, natural reaction is to protect where you got hit and wait to see how bad the pain is. His reaction was to laugh, hold where he was kicked, and thats it.
He could have covered his face, rolled around screaming, which is pretty much the bog standard reaction in these situations, he DIDN'T do that, he just laughed, yet people are reacting like somehow that is worse, when its far more truthful and not at all cheating.