I tested what you are claiming by projecting a shapefile in WGS84 to our Google Map projection and to your Google Map projection. You can see in the screen shot below that the shapefile in our projection (in red) is correct while the shapefile in your projection (in green) is incorrect. So I think that we have the correct projection string for Google Map.
This is the code I used:
winformsMap1.MapUnit = GeographyUnit.Meter;
winformsMap1.CurrentExtent = new RectangleShape(-14180662, 5547256, -7832569, 1777465);
winformsMap1.BackgroundOverlay.BackgroundBrush = new GeoSolidBrush(GeoColor.FromArgb(255, 198, 255, 255));
// Create your overlay and make sure to register and insert your own key.
GoogleMapsOverlay googleOverlay = new GoogleMapsOverlay(); //(@"Insert your key here!", @"C:\GoogleCache");
winformsMap1.Overlays.Add(googleOverlay);
// This sets the zoom levels to map to Googles. We next make sure we snap to the zoomlevels
winformsMap1.ZoomLevelSet = new GoogleMapZoomLevelSet();
//Google
Proj4Projection proj4 = new Proj4Projection();
proj4.InternalProjectionParametersString = Proj4Projection.GetEpsgParametersString(4326);
proj4.ExternalProjectionParametersString = Proj4Projection.GetGoogleMapParametersString();
ShapeFileFeatureLayer layer1 = new ShapeFileFeatureLayer(@"..\..\Data\austinstreets.shp");
LineStyle lineStyle = new LineStyle(new GeoPen(GeoColor.StandardColors.Red,2));
layer1.ZoomLevelSet.ZoomLevel01.DefaultLineStyle = lineStyle;
layer1.ZoomLevelSet.ZoomLevel01.ApplyUntilZoomLevel = ApplyUntilZoomLevel.Level20;
layer1.FeatureSource.Projection = proj4;
//Google test
Proj4Projection proj42 = new Proj4Projection();
proj42.InternalProjectionParametersString = Proj4Projection.GetEpsgParametersString(4326);
proj42.ExternalProjectionParametersString = "+proj=merc+a=6378137+b=6356752.31424518+lat_ts=0.0+lon_0=0.0+x_0=0.0+y_0=0+k=1.0+units=m+no_defs";
ShapeFileFeatureLayer layer2 = new ShapeFileFeatureLayer(@"..\..\Data\austinstreets.shp");
LineStyle lineStyle2 = new LineStyle(new GeoPen(GeoColor.StandardColors.Green, 2));
layer2.ZoomLevelSet.ZoomLevel01.DefaultLineStyle = lineStyle2;
layer2.ZoomLevelSet.ZoomLevel01.ApplyUntilZoomLevel = ApplyUntilZoomLevel.Level20;
layer2.FeatureSource.Projection = proj42;
LayerOverlay layerOverlay = new LayerOverlay();
layerOverlay.Layers.Add(layer1);
layerOverlay.Layers.Add(layer2);
winformsMap1.Overlays.Add(layerOverlay);
winformsMap1.Refresh();