My issue with the 3rd party/ someone else involved on the night theory is this:
Jeremy said that Ralph phoned him and told him Sheila had got the gun and was going beserk. That was sometime after 3am. For a third party to have been involved, that person would have had get in the house, not be seen by Ralph at anytime up to the alleged phone call to Jeremy, and at least get the gun off Sheila and then play an as yet unknown part in the killings. And then get out of the house again unseen.
The phone call that Jeremy tells us his father made, specifically excludes the possibility of a third party and specifically includes Sheila as the likely shooter.