Ah, the diagram is a bit unintentionally misleading there, the shaft length is specified in the 'build detail' table as
'Shaft type: 15mm x 12mm diameter steel then 14mm long 3/8 UNF external thread with 5mm left-handed internal thread.'
The overall dimension also includes the connection tabs on the rear of the motor hence the discrepancy, I will adjust it now.

How far long a blue wheel will sit on the shaft depends on the depth of the nut pressed into the wheel- I've had some blue wheels with a side lathed flush to the centre before the nut is pressed and some with nuts pressed straight in so the wheel over-hangs a shaft by at least 10mm more. I don't know if Robochallenge has standardised how they make them now so couldn't say 'this wheel will be positioned here' etc.

Also apologies on not getting back to you earlier Alex, I've just got back from holiday!

There's lots more in the pipeline, 18V GR01's will be released in under two weeks, bit of a website refresh coming and then more still, it's going to keep me busy!