Happy to report that the Bunnings 25.4mm tube works fine.
Middle Z assembly clearances are less than 1mm but no contact between Z axis tube and bearings or bolt heads.
Note that the bearing bolt holes in the Middle_Joiner and Middle_End parts are slightly offset from center. The Middle End parts can be turned around (print bed side facing outwards) to gain a little more clearance for their bearings.
I found it very important to make sure the Z axis pipes are exactly parallel (i.e the same distance apart at top and bottom). I spent a little extra time sanding the tube holders in the Z_Motor_Mount in order to get them perfect and achieve very smooth Z axis motion.
I also made a drum sander for my battery drill to aid in sanding the tube mounts for a good fit.