What i am having difficulty with is why in the descent/climb the turb n1 figure(which in the cruise provides near optimum penetration airspeed) cannot be selected.
This is probably more simple than you're trying to make it: in a climb or descent, airspeed is controlled by pitch. In level flight, it's controlled by N1.
In climb, you select the climb N1, and pitch is controlled simply via FMC or MCP SPD. In our case, 280/.78. This is just like in a Cessna 172 - max power, pitch for Vy, for example.
In descent, power is of course at idle and pitch is controlled by airspeed.
The Turb N1 is only for level flight (hence CRZ page) and is power setting to get you to the Turb Penetration Speed. It's never "selectable" and doesn't show on the N1 page.