A bogus spiritualist and an amateur actor hope to con a wealthy woman out of $10,000 by locating her sole heir — a nephew given up for adoption under shady circumstances — but find they are in deep water as the nephew turns out to be a kidnapper who’d rather not be found

This one is intriguing. But it doesn’t quite highlight the stakes enough. The nephew doesn’t want to be found right? So is he himself a much harder core criminal, meaning the fake spiritualist and the actor have bitten off more than they can chew and now are faced with life threatening retaliation? Or perhaps the nephew is even more of a cunning criminal than they are, and he outwits them at their own game. Maybe the nephew is an undercover cop and they blow his cover. There are tons of ways this could go. I think some hint of the direction of your movie will be helpful in the logline.

>>IMDB one-liners... are used to sell to an audience whereas a logline is used to sell to a producer

An important distinction. And some of the IMDB one-liners are poorly written; I wonder where they get them.

I'd never call those IMDB one-liners a logline. VERY different purpose! The one-liners are used to sell to an audience whereas a logline is used to sell to a producer. That's my approach anyway.