It's all too coincidental. LB goes missing. Who knows - if she died, maybe it was some kind off accident (drugs, or other). If he didn't plan it in advance he wouldn't have the incinerator on hand, but ordered one. I would think there would be no other logical reason for him to buy an incinerator. He doesn't have animals. Someone mentioned the swamp may be a place to "store" someone who met an unfortunate fate. Rich boy then places an order, and when the incinerator arrives he does the job.
Then, when Daddy says he is getting married again, Rich boy doesn't want to share and DM realizes he already has experience in this department and so he takes care of his Dad. Or perhaps Dad came across or found out about the incinerator and he realized son was up to no good. Only way to keep Dad quiet is to make it look like suicide.
One question. Why is it that a rich kid, who likes to have fun and party, lives with Dad? How come he didn't live somewhere more young and hip? Living with Dad, really? Does Daddy tell him to make his bed and do the dishes? The only reason why kids usually live with their parents when they are that age is to save money but he has lots. Also, why would he want to live there after his Dad shot himself there? Isn't that strange too?