Nevill's Will specified that after certain bequests, the main residue of his estate would go to Jeremy, so there was no advantage in killing Nevill after June. In fact, some people have said he would kill Nevill first to take out the main resistance. I haven't seen June's Will, so I don't know what it said.
Let's say that the plan went wrong, and Nevill wasn't killed or incapacitated in the bedroom. He went down to the kitchen for some reason and Jeremy followed him. Would Jeremy really be in such a rage that he beat Nevill rather than kill him straightaway in some kind of revenge?
I can't read anything into where Sheila would have shot herself.
A common misconception is Nevill left everything to Jeremy. In fact he bequeathed his liquid assets to his wife with £10000 going to his daughter, the farming business went to Jeremy, though I doubt Nevill in his heart of hearts really felt his son was going to persist with it. June left her share Osea Road to Sheila along with an adjoining field, to be managed by James Carr, the residue of her estate bequeathed to Jeremy.
Of course with five victims dying at the same time the stipulations of the will were in effect made meaningless, as Jeremy would inherit everything.