The coupling "11" is to allow fitment of the rod from the rear- as the front "cup" for the gear lever is larger than the ID of the shift tube, the front section has to be fitted seperately. The rear "T" section which fits into the gearbox coupling is also larger than the ID of the tube, so no, you can't replace the rearmost shuttlecock bushes with the gearbox in situ, the whole shaft- not including the front section "10"- has to be pulled out from the rear.
From an old post of mine:
"Gearbox input shaft (31)- linkage rod connector (fork connector) Keep the gearbox in. unbolt coupling, pull gear lever back to seperate.
"Shuttlecock" bushes (13, 16, possibly another 16)- You cannot replace the rear 2 (or 1, depending on the age of the bus) without removing the gearbox. You CAN replace the front one- disconnect the linkage rod as if you were changing the fork connector, disconnect the front linkage rod, push the big 'un through until you see the front bush.
Front rod bush (19)- on an early, that's a wee slip of the thing. Easy to change, take the gearstick out, undo the front coupling between the front rod and the main rod, take it out and change the little bush inside.
And that's all of 'em 
Once you've done that, do yourself a favour and fit one of those £5 quick-shift kits. Cheap as chips and make a real difference to the feel, plus they keep everything looking stock."
One thing you can do to improve the feel as well- the front shift bush is a sloppy fit on the pin. I 3D printed one, but you can turn one down from a PTFE/Teflon tube. Dimensions are linked in this thread:
http://forum.earlybay.com/viewtopic.php?f=4&t=78292
Makes a HUGE difference.
