As I said if Jeremy Bamber didn't do it, he wasn't in the house to know if there was a third party or not. Sheila isn't there to tell us what happened either.
Did someone hire a hitman? Were there other family members involved?
I concur with your statements.
Re your questions. It's easy enough to pull 'stuff' out of the ether, but things of this magnitude don't JUST happen. A hitman? Well, for £2000 you're not going to get a very professional one. A fact exhibited by the fact that he goes to do a job without his own 'piece', relying, instead, on the less than highly efficient weapon available. Apart from which, most hitmen don't like the thought of killing children. Then there's the problem of how this mysterious hitman enters and exits -not so much a problem if it's a family member- and where they'd hide their getaway vehicle. What about motive? The hitman (presumably someone unknown to, and not knowing, the family) just shoots everyone -as each one dies, Jeremy's inheritance grows. If it's a family member, they'd surely have guessed that, unless the Wills said anything contrary, Jeremy would inherit. How would that benefit the family?
You've clearly put a lot of hours into what you've produced, but at the end of it, had I needed convincing, you'd have convinced me that Jeremy -and only Jeremy- was responsible.