Your original code did not work properly, because you send the coordinates to Google Geocoding API for reverse geocoding.
This service searches for information about the place by coordinates (approximate).
Usually it sticks to buildings, roads, etc.
So coordinate returned by this service can differ from coordinates of the request.

    private void OnMapClick()
    {
        Vector2 coords = OnlineMapsControlBase.instance.GetCoords();
        OnlineMaps.instance.AddMarker(coords);
        points.Add(coords);
        if (points.Count == 2) OnlineMaps.instance.AddDrawingElement(new OnlineMapsDrawingLine(points, Color.red, 3));
    }

Now this makes me wonder by my original implementation was so convoluted.  All I ever wanted was the latitude and longitude for where the user clicked.  My OP was what I came up with after searching the examples and docs, so clearly when I think I know how to use this asset, I really don't.

    private void OnMapClick()
    {

        double lat;
        double lng;

        // Get the coordinates where the user clicked.
        OnlineMapsControlBase.instance.GetCoords (out lng, out lat);

        Debug.Log ("mouse click lat/lon: "+lat+"/"+lng);

        OnlineMapsGoogleGeocoding.Find(null, lat+","+lng).OnComplete += delegate(string s)
        {
            try
            {
                Vector2 position = OnlineMapsGoogleGeocoding.GetCoordinatesFromResult(s);
                if (position != Vector2.zero) _marker = OnlineMaps.instance.AddMarker(position,"");
                else Debug.Log(s);
            }
            catch
            {
                Debug.Log(s);
            }
        };
    }

The marker lands close, but it's very random as to where it lands in relation to where I've clicked.  I'm using OpenStreetMap zoom 14 with Tileset.  I have already tried the GetCoord() using the Vector2 rather than double.   Do you see anything wrong with this code?  Does it sound like something you've seen before?   What could I be doing wrong?

EDIT:  Added more details above.

Thanks