IMO without a solid alibi you can't rule him out. Having an alibi that involves your girlfriend would be considered a soft alibi.
There was motive for him to turn up that night. God only knows at what stage Colin could have turned up. Were they all dead by that stage? In my theory the phone call to Jeremy (3:30 AM) could have been from Colin Caffell.
So you’re implying Colin’s girlfriend covered for him?
And hs neighbours who saw his van parked outside ALL NIGHT LONG.
And his family/friends who he spoke to on the phone from home?
All these people covered up for him.
And what was his motive? He didn’t want to take the boys’ on holiday? Sounds a strong motive

And how did he know Jeremy Bamber has oh so conveniently left the rifle out, loaded? Which, actually — he didn’t. He hid that rifle ready to kill them all.
And why would he alert Jeremy, trying to sound like Nevill, when he wanted to get away from the scene without being spotted?
Your suggestion is so ludicrous I can’t believe I’ve responded 😳