Yeah, using surface normals to move the offset points is correct - the way offset surface works.
I spent some time yesterday and this morning working on a smooth transition between offset areas. It’s difficult, especially handling multiple offset areas with different offset values. I finally got something that seems to work as intended, but more testing is required.
The big Eval component that generates the vector amplitude values is a fail in the sense it is hard wired and must be modified when the number of curves (offset areas) changes. 
I tried a different approach today that somewhat simplifies the task. Offset Surface is used with the default distance value (‘offD’ slider = 15) and is then used as the input to SDivide. ‘offB’ and ‘offY’ (offset values for blue and yellow curves) are modified to be relative to the default (-5 and +5).
The disabled Cull (yellow group) shows points in the transition areas, affected by the ‘Range’ slider (blue group). The Graph Mapper shape determines transition offset values.
I don’t know anymore if it’s correct and at this point I don’t care! Too much time spent… The result is smoother (and slower) if you bump the ‘U Count’ slider up to 200. Requires Rhino file Test1c.3dm.
variable_offset_2023_Aug17a.gh (32.2 KB)