Bit late but...(and in no particular order)...
1) The 'exceptional circumstances' bit did exist in the original advice, as I remember reading it at the time, so he is correct in that he did not actually break the rules. That he did "break the spirit" of the rules (as Laura K put it), is another matter.
2) To me, his reasoning does make sense, up to a certain point in the story - wife ill, high chance he will get/has got it, get to somewhere where they can self-isolate/lockdown properly and where there will be suitable childcare for son should both parents be incapacitated for any period of time, while they can, using the 'exceptional circumstances' angle if required.
An infected person going from London- a city with a lot of infected people and a lot of hospiitals to Durham then a town with few infected people and few hospitals- is literally the most dangerous thing you could do. Cummings was attending the science briefings. he must have known this.
There was nothing to stop him self isolating at home. Many families have done it in much smaller houses than the Cummings family. He made no attempt to source childcare locally, and his wife wrote a blog implying she was well enough to look after their child.
I think waht happened was they decided to go to Mum-in Law's Castle because they were bored.
3) Obviously the 'eyetest drive' is very difficult to swallow, although I would imagine if true, was a lot less dangerous in practice than it sounds (more a sort of test whether the strain of driving brings on any eyesight issues of the type experienced earlier, rather than I can't really see love, but I'll try out pointing the car in the right direction amidst the fog).
It's a lie. Why not drive two minutes if you are really worried.
4) Chances are that this trip out was a double header - eyetest mixed with a little trip out on her birthday and last day before heading back south as a relief after both being ill - though I think he might have been better acknowledging this rather than hoping the 'eye' thing would suffice, which it clearly was never goign to be.
Agreed- except there was no eyetest aspect
5) Can Mrs C drive, and if so, could she not have driven some if not all of the journey south?
Yes she can
6) What was stopping DC from working from home from his lockdown home in Durham for at least a couple of days/weeks....?
Another good question
7) If Boris was ill/incapacitated on the day DC chose to depart from London, what was stopping him contacting Dominic Raab (as Boris' nominated stand-in) to have the discussion, or is this just recognition that DC ranks above everyone else in the party, so can make his own decisions.
He's allowed to do as he likes despite being unelected- see also his press conference in the Downing Street Garden for whihc he was 30 minutes late
8) Following on from (2), I wonder what would have happened had he done the same, but that it had been acknowledged as such by either himself or the party shortly after. What, I wonder, would have been the impact of referencing the 'exceptional circumstances' clause at that time? Could the govt have issued some clarification of what those circumstances could be and how they should be dealt with by the public in those situations?
They could. They don't want to because they have only contempt for the likes of us.
9) I also wonder what the value of him using the words 'sorry', apologise, or regret' at some point in the conference would have been. Would he have got an easier ride subsequently and the world move on to the more pertinent things being announced, or would it have just fueled the 'guilty, so must be fired lobby'?
See above.