Thanks again for all the help.
I think I've figured the workaround for Extension Tools "extrude edges by rails"

The trick to getting rid of artefacts at junctions seems to be:

1. With the top and bottom quarter-pie profiles, go to the end of each curve. Make sure the final segment is parallel to x or y axis. In other words, counter the default sketchup behaviour to place equal sized segments for a circle, with segments adjacent to axes at an angle. Make the end segments of profile curves are squared off instead.

2. Divide the long sections of the rails into many segments (I used 40).

Result: