OP, they’ve vastly underestimated you. And Mumsnet.
Let them keep digging the hole, in fact hand them shovels and drinks. I’d be sending communications that I knew would be deleted, sidelined or doctored. Very reasonable communications, easily traced, that feed into the ‘sunk cost fallacy’ that they’re desperately trying to salvage.
The more staff they co-opt into this little nonsense conspiracy, the bigger the shitshow when it all comes out. You know who will blink first? One of this staff group who is only tangentially involved and figures out he/she is in line to take the blame ‘unfairly’. Watch their loyalty to CF evaporate, as this gets closer and closer to a sackable offence.
Make sure you read over the school’s mission statement(s) and use the language when you (finally) get to speak to the Head. There’ll be something there about accountability and transparency. Ask how exactly this dishonest behaviour by staff is acceptable role-modelling to students, especially given that this debacle is playing out right in front of them.
I’d probably greet the Head with a phrase like ‘Sorry, not sure if I should curtsy. You’re more protected than the Royals, when it comes to being granted an audience!’ And I’d use a very friendly tone, big smile. And be ready with how many times you asked to speak to him directly. I’m counting at least 5? Say that you believe he has an Open Door policy, particularly to resolve these problems before they escalate. But that his staff (multiple) have been raising the drawbridge to prevent anyone getting to his open door, and they’ve been doing it to cover their behinds. That their loyalty is quite obviously misplaced. It’s not to him, students, the school or the truth.
Here’s hoping the Head is as honest and capable as you say...