Well-argued but he doesn't need to tell anyone his plan of staying the night at White House Farm until the last moment, maybe surprising Nevill, throwing him off guard and walking back with him to the ingress. He has already blabbed to Julie on the telephone from Bourtree Cottage that he had been thinking about the crime all day, and "it's now or never.."
You may be right. By all means, he may well have told them he was staying the night. Who knows? Not sure what difference it makes, and I personally think it unlikely anyway.
Three circumstantial reasons I find it unlikely, apart from Len Foakes' evidence:
(i). [To repeat] I think Jeremy would not want to take the risk of a third party discovering that he was a guest at the farmhouse that night. If he planned this, he must have thought about it. In the event, June and Sheila mention nothing to Pamela about Jeremy staying. I appreciate it may be that only Nevill knew, but remember that Dorothy Foakes had heard Jeremy drive off at 9.30 p.m.
(ii). If Jeremy is the killer, and if you otherwise accept Len Foakes' evidence about what Nevill was doing, and if you accept my earlier posts about the need for Jeremy to go to and from the farm on foot, then Jeremy's time bracket for committing the murders is roughly 11 p.m. to 2.30 p.m. Contrary to what Caroline says, it makes sense that Jeremy would leave this as late as possible. I think, if Jeremy did this, the family were killed at some point between 1.00 p.m. and 2.15 p.m. He wouldn't wait around at the farmhouse for hours, even if it was under the guise of being a guest, nor would he want to risk a melee. (The fact there was a melee does not detract from this point).
(ii). The fact there was a melee (or so it appears) indicates that if Jeremy was in situ, he has not executed it very well. He has not waited until his parents are asleep, which is strange. After all, if his parents heard something, in all seriousness wouldn't Jeremy just shout: 'Oh, don't worry, just me!' They would not investigate. Jeremy in situ is in the perfect position to wait until the perfect moment. The fact there was a melee suggests to me that the killer was either a distressed individual running amok or an intruder - the latter of course would have to be Jeremy.
None of this is to belittle Caroline. I reiterate: it's an interesting theory.