I read the 'Why Barnard Castle?' blog and it has a convincing ring to it but, firstly, why on Easter Sunday? Seems a funny time to be making deals and secondly, as grumppa says above, the Skripal poisonings. I think he called it completely wrong on that.
I know he's a former diplomat but it's not necessarily a recommendation.
The only conspiracy theory that's grabbing me at the moment is the capture of the tory party by the extreme right, with links to some objectionable US white supremacists and power maniacs, to Russian oligarchs with criminal tendencies and to disaster capitalists, with the objective of milking the UK in the chaos that will follow a no deal Brexit. I'm not sure that Cummings is totally part of that, he has his own agenda of destroying the civil service and the institutions of the state which fits conveniently with the far right agenda at the moment.
The thing about this 'conspiracy theory' is that it can all be seen out there, in plain sight, in the writings of the people involved.