Any question answered...

Have an 18T EAI if you want to borrow?

I've just never though much about cadence, just inches.
However for long flat rides I imagine its better to focus cadence(?).

Usually do a 2+ hours at 95. Thought that for 6+ hours 85 would be better, that there would be less fatigue. However read that higher cadence is better for blood flow, reducing fatigue.

Maybe gear up and drop cadence for the first few hours of the Dynamo and see how it feels? I suppose it's all personally preference.

Noiser, I change my gears quite frequently but generally found that if I was happy about town I'd be happy on a longer ride, for me this sits at about 68GI, you're probably asking so you can decide on a gear for the dun run?
I'll be on 71.5 just because it's the most reasonable thing I can get with my lack of cog choice.