You do well to follow the money. According to Felos at the beginning of Feb. there was about $60,000 left in the trust fund for Terri. That's not nothing, at least where I come from. It might be to someone who lives in a $400,000 home, but to me it's plenty.
Next, I think it is extremely likely that there is a life insurance policy on Terri from at least the days when she worked for Prudential, before her collapse. Big companies like that often have perks for even the rank and file employees, and I'm sure life insurance, perhaps even at reduced rates, would be one of them.
I also suggest you consider that since Michael's girlfriend is in the insurance business, it's not too far out of reach to think that he could have gotten a policy out on Terri through Jodi.
On top of that, I'm guessing that there's a good bit of money to be made off of book and movie deals, knowing the way Hollyweird is.
Michael has plenty to gain financially even if there were absolutely nothing left to inherit from trust funds, but I'm pretty sure there's enough in those to give Michael motive.
Absolutely impossible. I defy you to secure a life-insurance policy for someone in Terri's condition. Good luck, but you'll never find an underwriter.