@Roch
Room 101?? Was he being threatened with rats when he said that?
Anyway, I don't put much stall in Stan Jones' words about the phone/kitchen in that COLP interview, simply because he is referring to the state of the phone and/or kitchen post-incident. It is common ground that the phone was working - which, for me, is the important observation.
The point is: There's no phone in the master bedroom and the phone in the upstairs office is behind a locked door. They don't have mobile phones at that point in time. On this basis, if we assume Jeremy is telling the truth, then I am deducing that the call to Jeremy was made from the kitchen. This also makes sense because of where the rifle and magazine are. Sheila must go downstairs at some point.
The twins aren't downstairs, they're upstairs. Nevill is making the call downstairs, not upstairs, and he has to make the call before anybody else is shot, otherwise he would dial 999, surely? He must also make the call before he is shot, or again he would attempt to dial 999 and he certainly would not be speaking to Jeremy.
This means that in a Sheila scenario, there must be a stand-off between Nevill and Sheila in the kitchen, and more than likely, Nevill makes the call to Jeremy with Sheila present. The more I think about it, the more I believe he rang Jeremy because it was the first thing that popped into his head in a panicked situation. All sorts of things would have been running through his mind: his daughter, the safety of the twins and June, his firearms certificate and his reputation as a magistrate, obviously with the twins at the forefront of this mind.
It could be that your scenario fits all this in that Sheila goes downstairs and intends then to return upstairs with the rifle, but Nevill stops her in the kitchen. She then starts ranting and raving and that may have gone on for some time, before a confused and worried Nevill rings Jeremy, maybe as a way of keeping her in the kitchen, but the ploy backfires as Jeremy's involvement only makes her angrier.
As for Sheila's motivations, I am not sure because I am unsure whether loss of custody would have bothered her. That might sound like an odd thing to say, but she was ill and may even have welcomed Colin's offer to take on the burden on the understanding she had contact. On the other hand, Dr. Ferguson seemed to think a threat of loss of custody would have been cataclysmic for her psychologically.