Need travel insurance but diagnosis is less than 6 weeks ago

As far as I'm aware the 6 week stance is pretty widespread. If you haven't tried already a comparison site might be useful. I'd suggest also trying albert&eddie (strange name I know) if you've not heard of them, they're part of the same group as Insurancewith and I got a good annual quote with Holidaysafe from them.

The question of when a diagnosis is official would, I believe, be when your specialist confirms the diagnosis and for insurance purposes if you had a claim the insurer may well want to see written confirmation from them to prove the date of official diagnosis.