So, when it comes to speed vs stability vs comfort, what are peoples thoughts?

29 x 2.1 Vulpines

or

700 x 40c Hurricances

Decisions, decisions!

I'd say go out on both tyres (if you already own them) down a "rough" gravel track in the dark, preferably with some good descent.
Then decide what you'd be happy with for those 10 hours

What Jack said - test stuff in the dark. If it works in those conditions, then they'll be fine in the day. Think about general fatigue and slow reaction times in the middle of the night and work out which tyre gives greatest confidence/ margin for error. I'm pretty sure I'll be on something around 2", not 35-42c.

I think the larger ones will offer a little more confidence why you start getting tired, and will be a little more comfy on the old buttocks.

I'll be on either 2.0" or 2.2" specialized fast tracks.
The 2.0's are looking shabby on the sidewall now so will likely be on the larger tyres.