As SIZE is a critical issue here, What are the "exact" dimensions of the
cockpit. Seat pan at it's narrowest and Widest? Seat back to Rudder
pedals at furthest extension. Cockpit Shoulder width? Is the Diana 2
cockpit different from the original SZD-56?



The height is 31" when measured at an angle of 25 degree.
The ASW 24 is around 34"
HP 18 is 27"
my modified HP18's are 33"

If you are a person with no neck or back problem and if all the other
things are ergonomically sensible you should be able to fly for 5 plus
hours.
I do not know what the cumulative effect would be, to be relatively steeply
reclined and the neck slightly extended, after 12 days in a world contest.

There is one other consideration. In both, the ASW24&27
fuselage nose is drooped relative to the centre line for aerodynamic reasons
but will aid in achieving a better view and a more upright position.
The Diana does not have that feature or if it does, it is hardly noticeable.
I noticed in the comments that the pilot alluded to the fact he has no
obstruction of the view any more. That would indicate the angle at which
the wing is set may have changed to make this possible.

Regards
Udo