Yup! Watched it. What did you think
belboid ? I thought it was an interesting ending when it steered away from the predictable trying to work out who it was.
But.... IMO it wasn't Suspect 4 (the copycat), and I thought Suspect 1 was highly unlikely as well. That younger bloke (her 'memory man') who befriended her has pushed the old woman into creating a story around it to deal with the fact her ex-husband had clearly been dodgy and lied to her in other ways. He was very creepy too, he was taking advantage of her vulnerability I think.
Barbara and then the Lynn Doyle Cooper (obviously...) that was an uncle of Marla were the most likely candidates by far I thought. Marla passed a polygraph and she has some fairly convincing stories around it. Both seem to have had the capability (pilots and parachuting) as well, although apparently the FBI thought he wasn't that experienced as a parachutist. Maybe Barbara needed the money for her gender re-assignment surgery (shades of a Dog Day Afternoon)? I'd have thought given that she'd have been an easy one to rule in or out as well.
I'd put a bit of money on it being none of them though, and it actually being some unidentified person who then died in the jump, maybe by landing into water, and then that explains the money recovered years later washed ashore.