As for sidewalks, we don't model them in our corridors for the reasons you mention, i.e. too many variations. We use an underlying xref with sidewalks and other non-essential-to-the-model items drawn in. That saves a lot of time. Intersections are always done with the Intersection Wizard, which is a great time saver.
Parking bays I have had partial success by attaching a LaneOutsideSuper section to the ETW of the main center assembly that targets an alignment following the parking bay. Where there is no parking, the attached lane is zero length. When bays appear, the attached lane assembly grows out to follow the alignment, then returns to zero as the bay recedes farther up the road. The problem here is that curbs look funny at the bay radii because they remain perpendicular to the centerline alignment. An offset assembly would fix that, but you need a separate profile for them and that starts to add time back into the process.
Ditches are a problem where there are lots of driveways (and, hence, culverts), as are driveways themselves. I usually model the driveways using feature lines, not assemblies. Ditches are assemblies, but as always you must start and stop them often, which means lots of regions until they come up with a better idea, like yours.
