Not sure if you ever got this working but since your code helped me get adding shapefiles... I figured I would post how I did it.
// Variables
string fullpathToShapefile = "C:\Shared\sample\sample.shp"
string shp_path = Path.GetDirectoryName(fullpathToShapefile); // This will be "C:\Shared\sample"
string shp_name = Path.GetFileNameWithoutExtension(fullpathToShapefile); // this will be "sample"
Type factory_type = Type.GetTypeFromProgID("esriDataSourcesFile.ShapefileWorkspaceFactory");
IWorkspaceFactory ShpWksFact = (IWorkspaceFactory)Activator.CreateInstance(factory_type);
IFeatureWorkspace FeatWks = (IFeatureWorkspace)ShpWksFact.OpenFromFile(shp_path, 0);
IFeatureClass FeatClass = FeatWks.OpenFeatureClass(shp_name);
IFeatureLayer FeatLayer = new FeatureLayerClass();
FeatLayer.FeatureClass = FeatClass;
Hope it helps
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