What do you mean by the quadrant shifter being a pain to shift? Is it because of the top tube position or were they just not mechanically sound and efficient?

As for handlebars, I understand the major taylor bars that come with the clubman are not comfortable for street riding being as they are track bars. Can you recommend a suitable clubman style drop bar replacement?

And yes, I would use the coaster brake together with a front brake. Can't wait to see your new clubman build!