If it was part of his cunning plan it was pretty pointless really. Everyone was dead so why bother to say he loaded the gun? There was no reason to mention any gun being left out loaded. Guns were always left out round the farmhouse, it's the way farmers lived in those days.
Thought guns were usually left in the locked gun cupboard.
Jeremy left the house at 10 that evening. He would be hopeing that a tired Neville will not bother putting the loaded gun away. The 5 people were in the house. At the same time. No wonder Mugford says Jeremy said 'tonights the night' & 'it's now or never'.
Or as you said, perhaps there were lots of guns accessible.