To do what think you want to do with out resorting to SAC, I would draw a FL at the existing curbline. Use LinkOffsetOnSurface, set to be omitted or with an code that would not be used in surfaces, inserted on you back of curb targeting the FL and existing grade would give me the slope between those points. A LinkWidthAndSlope at the same insertion point as the LinkOffsetOnSurface, also omitted, with the width set to distance to the mid point of the sidewalk and the slope inherited from the LinkOffsetOnSurface (set under assembly properties dialog), with a second omitted width LinkWidthAndSlope set to half the width of the sidewalk as negative (back towards the center) at the reverse slope of the walk to a MarkedPoint sub as the inner edge of the sidewalk. Using a LinkToMarkedPoint and the sidewalk and another LinkOffsetOnSurface (targeting the FL at the curb line) should get you pretty close to what you want. This assumes you are keeping the parking curb, if not just add your curb to the last link. You could target a FL with the first LinkWidthAndSlope width target to vary the offset of sidewalk centerline.
I think, if you know the actual slopes you want to use, you could use LinkSlopeBetweenPoints, omitted, to establish the location of the sidewalk, and having the first link in you assembly inherit its slope and offset from the first link In the LinkSlopeBetweenPoints sub.
