K52 - I'm no-one here would disagree that in the event of inadvertant entry to IMC then a climb to safety alt as you describe is the only option. Indeed, some of the evidence (notably relating to engine speed) indicates that this might be exactly what the crew of ZD576 were trying to do. Can you prove otherwise?
As to your other points:
The BOI further concluded that it was likely that the Captain "made an error of judgement in the conduct of the attempted climb over the Mull"
The key word here is
likely. Again few would dismiss this as a possibility, but where's the evidence?
OC RAF Odiham stated that " the root cause of this accident was that the crew, while operating in poor weather and close to high ground, flew the aircraft into the ground."
OC RAF Odiham was no more in the cockpit that day than you or I, so how on earth can he know for sure?
" This sortie was planned to be a low level daytime passenger carrying transit flight flown clear of cloud under Visual Flight Rules. Yet when the aircraft crashed it was flying at high speed, well below safety height in cloud in direct contravention of the rules for flight under VFR or IFR."
These are the facts, however there's nothing in that sentence (or the evidence) that proves conclusively, beyond doubt, that the crew were the only cause of the crash.
I say again - there is no evidence to support the conclusion of negligence.