My sense is they have not yet arrested the person who actually committed the murder. Remember: Dellen was driving. It is likely the second suspect who committed the murder.
Police have not arrested AM or SS. Never even said they were POI. AM suddenly trying to act normal on twitter and FB. I am guessing police have been working both for days, possibly even before Millard's arrest. Just need one to turn and cut a deal. Millard wasn't rich. He had means but he also had heavy monthly costs. But, to AM and SS, he would look rich and be responsible for their lifestyle.
The hanger and equipment went on sale in March. The farm was probably bought to ensure that as the business transitioned, they would still have a runway, which would enable their illicit activity. Getting a burn phone a few months ago suggests these guys were going deeper in to a criminal world, as they bonded with tats and haircuts. They would see each other as loyal to one another and growing business partners.
It doesn't take LE long to break a group of fools like this apart. Hold back the murder charges as long as possible as leverage in cutting a deal as you get them to turn on one another. LE were likely led to the body. By one of them. Murder charges coming for more than one person. And then they all go to jail for 25 years, hopefully. But, a poor little girl who lost her father this week will not yet have turned 30 when they are released. Does not seem just or right. JMO.