I've been watching Columbo since it aired but Old Fashioned Murder baffles me. The ending was anti climatic for sure.
So Columbo has the brother on tape doing inventory on the museum pieces. The evidence is that the piece had to have been taken after the robbery and planted in the niece's closet. Since she had no idea of its value it had to be Van Patten?
Is that the evidence?
Ruth mapped out an elaborate story about the museum piece being a stolen item two weeks prior to the murder. She did that to incriminate Janie. It was obvious that the piece was planted by Ruth because the item was actually there during the inventory, placed in Janie's closet (Columbo saw Ruth there before he interviewed Janie), and as you stated Janie had no idea what it even was.
Circumstantially, as everything else is too, he also had the point about the lights being turned out. Obviously, no killer outside of the museum staff would have cared about the electric bill.
Ruth was willing to admit to the murder because she didn't want Janie to know the truth about the whole Peter Brandt thing or that Ruth had tried to frame her.
I hope that's helpful. That's as much as I remember.