Actually you're saying that - I was suggesting something else. Jeremy didn't just indicate that Sheila had a gun, he also indicated that 'She had gone crazy'. The disarray may not have been set up to look like an altercation but simply to show that someone had indeed 'gone crazy'.
So you are suggesting some of the damage was done during the altercation and it was enhanced to suggest she was making a mess before she started shooting? That makes it less likely for June to have stayed in bed. To stay in bed as she hears dishes flying makes no sense.
If there had been no altercation in the kitchen but rather everyone died in their rooms and I could buy him perhaps making the error of staging a mess afterwards to try to suggest she was flipping out (I say mistake because it defies them being found in bed so ruins the narrative he was trying to present) but given the altercation I don't buy it. Especially since breaking the lamp and scratching the mantle is not something that would happen from staging a mess.
When he says she went crazy I take that as meaning was having delusions not she was tossing things around in a fit of rage. If she is pushing chairs over and so forth that puts her in an easy position to be disarmed. Disarming someone holding a weapon with both hands and aiming it is harder to disarm though if they are close enough you can do it. The easiest way is to get so close the rifle can't be effectively aimed at you, (side step the rifle) then grasp it with both hands and try to maneuver the stock into the jaw of the gunman.