Programmatically Start Play Mode in the Editor

To launch from a menu item you would

[MenuItem("Play/PlayMe")]
public static void BuildAssetBundles()
{
    EditorApplication.isPlaying = true;
    EditorApplication.NewScene();
    GameObject go = new GameObject("PlayMode");
    go.AddComponent<PlaySomeMonoBehaviour>().WhatToDo =
        PlaySomeMonoBehaviour.PlayMe;
}

In the PlaySomeMonoBehaviour class

public const string PlayMe = "PlayMe";

void Start() { switch (WhatToDo) { case PlayMe: StartCoroutine((YetSomeOtherClass.DoSomething())); break; } }

YetSomeOtherClass is just another MonoBehaviour manipulating GameObjects.