Your confusion probably has nothing to do with anything "senior"!
The slot, of course, depends on how thick the head of your glassed and finished CB ended up.
Mine was 35mm. It was a kit CB foil which started as two halves, each of 18mm. Vacuumed-bagged boards will be less, maybe down to 32 -34mm.
The centerboard shoulders should end up being around 32 - 35mm after glassing.
Add a 4.5mm gap space on either side plus 9 mm case sides on each side and that will result in a sized to fit centerboard slot on the hull of 59 - 62mm.
The trunk assembly should fit through the hull CB slot so that the bottom of the trunk is flush with the hull bottom. Leave the logs long at the bottom to be trimmed later. You'll want the log tops cut flush so the seat-top can be dry-fit laid down during assembly of the Bulkheads 4 - 7 to ensure they remain nicely vertical during glue-up
The CB trunk logs should be 38mm fore/aft and a width of 41 - 44mm (sized to fit)
You say "The plans show an opening width of 68.5mm". I don't find that on my plans (rev Jan 1, 2012).
My Plans Sheet 5 shows the slot and says "Approx 56mm sized to fit constructed centerboard plus 4.5mm gap on either side". Maybe the plans need to be changed to read "Approx 62 - 67mm (sized to fit constructed centerboard plus 4.5 - 6mm gap on either side plus the 18mm for CB trunk sides)"
FYI, the CNC kits are now being shipped with CB slots cut to 67mm
I feel the 4.5mm on each side of the CB is too tight and probably should be at least 6mm. This would make the slot 62 - 65mm. See slot and gasket comments on the below thread on the subject at http://smallcraftadvisor.com/message-board2/viewtopic.php?f=3&t=1581
Hope this answer is more help than confusion to you.