I don't hold any MGM stock so I don't follow it. But looking over the charts and checking the short interest on the stock I don't see anything overly suspicious in the mentioned transactions.
Let me point out that most executive transactions for corporations are preset or allowed at specific times. Which means Murren's transaction would have been previously known before the event at Mandalay took place. As far as Soro's shorts are concerned a $15 million profit on a stock with a market cap of $17 billion isn't out of the ordinary.
Also MGM stock peaked at about $34 in July of 2017 and has been cycling down since this high. So playing this stock short in a downtrend is not unusual.