How to call CreateProcess() with STARTUPINFOEX from C# and re-parent the child

I need to create a new process but so that it is a "child" of another process not the current process eg re-parent the new process.

The following have got me almost there .NET : How to call CreateProcessAsUser() with STARTUPINFOEX from C# and .NET : How to PInvoke UpdateProcThreadAttribute and http://winprogger.com/launching-a-non-child-process/

using System;
using System.Diagnostics;
using System.IO;
using System.Runtime.InteropServices;

public class ProcessCreator
{
    [DllImport("kernel32.dll")]
    [return: MarshalAs(UnmanagedType.Bool)]
    static extern bool CreateProcess(
        string lpApplicationName, string lpCommandLine, ref SECURITY_ATTRIBUTES lpProcessAttributes,
        ref SECURITY_ATTRIBUTES lpThreadAttributes, bool bInheritHandles, uint dwCreationFlags,
        IntPtr lpEnvironment, string lpCurrentDirectory, [In] ref STARTUPINFOEX lpStartupInfo,
        out PROCESS_INFORMATION lpProcessInformation);

    [DllImport("kernel32.dll", SetLastError = true)]
    [return: MarshalAs(UnmanagedType.Bool)]
    private static extern bool UpdateProcThreadAttribute(
        out IntPtr lpAttributeList, uint dwFlags, IntPtr Attribute, IntPtr lpValue,
        IntPtr cbSize, IntPtr lpPreviousValue, IntPtr lpReturnSize);

    [DllImport("kernel32.dll", SetLastError = true)]
    [return: MarshalAs(UnmanagedType.Bool)]
    private static extern bool InitializeProcThreadAttributeList(
        out IntPtr lpAttributeList, int dwAttributeCount, int dwFlags, ref IntPtr lpSize);

    public static bool CreateProcess(int parentProcessId)
    {
        const uint EXTENDED_STARTUPINFO_PRESENT = 0x00080000;
        const int PROC_THREAD_ATTR