That's just the point. We don't know how things developed so there's no point in anyone pretending they do know. Sheila could have taken the gun and opened fire upstairs while Nevill was outside collecting that last wagon load for all we know.
But how would this fit with JB's claim of a tel call from NB at circa 3.30 am?
I was rereading CC's wit stat which states when he stayed at WHF NB went to bed at 9.30 pm to 10 pm. As you've recently recited from CAL, LF saw NB on the farm at 10.30 pm. As it was harvest time I guess he would be up again early in the morning so it stands to reason he would want to get to bed ASAP after coming in.
I think he did go to bed as there's an indentation on his pillow and I think the housekeeper had been in that day. Either way I doubt the Bamber's were slovenly in that they didn't shake bed pillows and make beds on a daily basis if the housekeeper wasn't around.
If you read CC's wit stat he states as above re NB and that June went to bed after him and SC anytime. We know PB called at circa 10 pm and according to her SC was on her way to bed.
We know for sure June went to bed as her head was on her pillow when she sustained gsw's in the first episode evidenced by two bullets embedded in the pillow which correspond with wounds along with the blood stains.
I'm inclined to think the Bamber's put uncomfortable truths to SC re her future. Unable to sleep she either sought out NB or he was disturbed in some way by her moving about and the pair ended up downstairs.
I'll start a new thread about the possibilities of arguments/disagreements.