The fundamental object to know when you want to work with the shell is IShellItem. From here you can get other interfaces using the BindToHandler method.

To create the initial IShellItem from a path, you can use SHCreateItemFromParsingName.

You should use normal COM interfaces definitions instead of trying to build v-tables by hand. Here is a sample code that seems to work:

class Program
{
    [STAThread] // don't forget STAThread, most shell methods require that
    static void Main(string[] args)
    {
        string path = @"d:\kilroy_was_here\test.htm";

        int hr = SHCreateItemFromParsingName(path, null, typeof(IShellItem).GUID, out IShellItem item);
        if (hr != 0)
            throw new Win32Exception(hr);

        hr = SHAssocEnumHandlers(Path.GetExtension(path), ASSOC_FILTER.ASSOC_FILTER_RECOMMENDED, out IEnumAssocHandlers eah);
        if (hr != 0)
            throw new Win32Exception(hr);

        do
        {
            eah.Next(1, out IAssocHandler handler, out int fetched);
            if (fetched != 1)
                break;

            handler.GetName(out string name);
            Console.WriteLine(name);
            if (name.Contains("firefox")) // open with firefox, for example
            {
                var BHID_DataObject = new Guid("b8c0bd9f-ed24-455c-83e6-d5390c4fe8c4");
                var dao = item.BindToHandler(null, BHID_DataObject, typeof(IDataObject).GUID);
                handler.Invoke(dao);
            }
        }
        while (true);
    }

    [Flags]
    public enum ASSOC_FILTER
    {
        ASSOC_FILTER_NONE = 0x00000000,
        ASSOC_FILTER_RECOMMENDED = 0x00000001
    }

    [Guid("973810ae-9599-4b88-9e4d-6ee98c9552da"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    private interface IEnumAssocHandlers
    {
        [PreserveSig]
        int Next(int celt, out IAssocHandler rgelt, out int pceltFetched);
    }

    [Guid("f04061ac-1659-4a3f-a954-775aa57fc083"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    private interface IAssocHandler
    {
        void GetName([MarshalAs(UnmanagedType.LPWStr)] out string ppsz);
        void GetUIName([MarshalAs(UnmanagedType.LPWStr)] out string ppsz);
        void GetIconLocation([MarshalAs(UnmanagedType.LPWStr)] out string ppszPath, out int pIndex);
        [PreserveSig]
        int IsRecommended();
        void MakeDefault([MarshalAs(UnmanagedType.LPWStr)] string pszDescription);
        void Invoke(IDataObject pdo);
        void CreateInvoker(IDataObject pdo, out /*IAssocHandlerInvoker*/ object invoker);
    }

    [Guid("43826d1e-e718-42ee-bc55-a1e261c37bfe"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    private interface IShellItem
    {
        // here we only need this member
        IDataObject BindToHandler(IBindCtx pbc, [MarshalAs(UnmanagedType.LPStruct)] Guid bhid, [MarshalAs(UnmanagedType.LPStruct)] Guid riid);
    }

    [DllImport("shell32", CharSet = CharSet.Unicode)]
    private extern static int SHCreateItemFromParsingName(string pszPath, IBindCtx pbc, [MarshalAs(UnmanagedType.LPStruct)] Guid riid, out IShellItem ppv);

    [DllImport("shell32", CharSet = CharSet.Unicode)]
    private extern static int SHAssocEnumHandlers(string pszExtra, ASSOC_FILTER afFilter, out IEnumAssocHandlers ppEnumHandler);
}

PS: I used IDataObject from System.Runtime.InteropServices.ComTypes