Whilst much of what you have said is valid, as certainly wasn’t using the arguments in favour of Cameron and Osborne.
The first point (a) is seen as a joke now, because the author of the note realised after the event what a stupid thing it was to put in writing. The second point (b) about the OBR. Is a statement of fact. Governments finances are reviewed by the OBR and therefore are pretty much an open book and hence Starmer and Reeves are lying when they said things are worse than they expected. Things have, of course, become worse because of decision that have made in their 7 weeks in office. Come to think about it, that is what you have identified- the black hole is where the Ming vase was dropped and Labour’s costly hidden intentions became a reality.
Drawing attention to Cameron and Osborne is unhelpful to your argument as Starmer has said that his government will be different. A whole 7 weeks in power seems to suggest otherwise, same old lies same old croneyism- just a different set of croneys.