Exactly. They wanted the body found.
For reasons already stated, they must have figured on it being found relatively quickly. When that didn't happen, they must have wondered how events would unfold. I believe being left alone with the body in the house was one of the last things they wanted. If that happened, then they had to explain, and possibly stage, the return.
They could of course claim that the kidnappers had left the body in the WC, which is how the story was supposed to go anyway. Since that was the story they wanted police to believe, there is no advantage in waiting, might as well get it over with.
Even had the cops left them alone, with the body undiscovered, they would have had no options. They couldn't be sure the house was not under police surveillance, so they couldn't say the kidnappers returned JBR n the middle of the night. The cops would know no one had been there.
Basically once the 911 call was placed, the body had to be found. The story had to be that the killer/kidnapper "hid" the body in the WC hoping it wouldn't be found before ransom was collected - or hoping for the so-called "kidnapping gone bad" theory to be floated.
If there was a plan I think they knew they could not proceed with the next step until the body was found.