Allah (swt) addresses himself in Quran as I and WE??

Part of a lengthy narration in which Imam Ali (as) answers to various objects raised upon the verses of the Qur'an:

واما ما كان من الخطاب بالانفراد مرة، وبالجمع مرة، من صفة الباري جل ذكره، فان الله تبارك وتعالى اسمه، على ما وصف به نفسه بالانفراد والوحدانية، هو: النور الأزلي القديم الذي ليس كمثله شئ، لا يتغير، ويحكم ما يشاء ويختار، ولا معقب لحكمه، ولا راد لقضائه، ولا ما خلق زاد في ملكه وعزه ولا نقص منه ما لم يخلقه، وانما اراد بالخلق اظهار قدرته، وابداء سلطانه، وتبيين براهين حكمته، فخلق ما شاء كما شاء، واجري فعل بعض الأشياء على ايدي من اصطفى من امنائه، وكان فعلهم فعله، وأمرهم أمره، كما قال: " ومن يطع الرسول فقد أطاع الله " وجعل السماء والأرض وعاء لمن يشاء من خلقه، ليميز الخبيث من الطيب، مع سابق علمه بالفريقين من أهلها، وليجعل ذلك مثالا لأوليائه وامنائه، وعرف الخليقة فضل منزلة أوليائه، وفرض عليهم من طاعتهم مثل الذي فرضه منه لنفسه، والزمهم الحجة بأن خاطبهم خطابا يدل على انفراده وتوحده وبان له أولياء تجري أفعالهم واحكامهم مجرى فعله، فهم: " العباد المكرمون لا يسبقونه بالقول وهم بامره يعملون " " هو الذي ايدهم بروح منه " وعرف الخلق اقتدارهم على علم الغيب بقوله: " عالم الغيب فلا يظهر على غيبه أحدا الا من ارتضى من رسول " وهم: النعيم الذي يسئل العباد عنه، لأن الله تبارك وتعالى انعم بهم على من اتبعهم من أوليائهم. قال السائل: من هؤلاء الحجج ؟ قال: هم رسول الله، ومن حل محله من أصفياء الله الذين قرنهم الله بنفسه ورسوله، وفرض على العباد من طاعتهم مثل الذي فرض عليهم منها لنفسه، وهم ولاة الأمر الذين قال الله فيهم: " اطيعوا الله واطيعوا الرسول واولي الأمر منكم " وقال فيهم: " ولو ردوه إلى الرسول والى اولي الأمر منهم لعلمه الذين يستنبطونه منهم قال السائل: ما ذاك الأمر ؟ ال علي عليه السلام: الذي به تنزل الملائكة في الليلة التي يفرق فيها كل أمر حكيم، من: خلق، ورزق، واجل، وعمل، وعمر، وحياة وموت، وعلم غيب السماوات

والأرض والمعجزات التي لا تنبغي الا لله واصفيائه والسفرة بينه وبين خلقه، وهم وجه الله الذي قال: فاينما تولوا فثم وجه الله


Imam Ali (as) says,

"Regarding Allah (swt) addressing himself with singularity (I) sometimes and with plurality (We) sometimes, this from among the characteristics of Allah (swt). Verily Allah (swt) whenever He uses singularity (I) to address Himself & introduce his oneness, it refers to that, He is the very ancient and ever existing (eternal) light (Noor) and there is nothing like Him and He does not undergo any changes. He orders whatever He wishes and chooses whomever he wants. And Here is none to stop His orders and none to reject His decisions. And whatever He created did not increase his kingdom and His might, and did not decrease anything from it what He did not create. Surely He choose to create so as to display His power and demonstrate His authority and to makeclear the proofs of His wisdom. So He created whatever He wished and However he wished."

And He carried out acts of some of the things upon the hands of those whom He chose from His trustees. So their act is His act and their order is His order, like the saying of Allah (swt), "And Whoever obeys the Prophet, then he has obeyed Allah (swt)" (4:80). And made the sky and the earth containers for whomever he wished from his creations, so as to differentiate the bad from the good, although he from before had knowledge of the people of these 2 groups (bad and good) (then they do about themselfs). So as to make that an example for his friends and trustees. And He defined to the creations the preferred status of his friends and made obligatory upon them their obedience just like he made obligatory his obedience. 

And He proved to them that he gave them speech indicating his uniqueness (individuality) and oneness, and that He has custodians (Awliya) whose actions and judgements/decisions are on the same path as His deeds, indeed they are his Honoured Slaves, They speak not until He has spoken, and they act on His Command, and strengthened them with Ruh (proofs, light and true guidance) from Himself.

And He made known to the creations their authority upon the knowledge of the Unseen (ilm al Gaib) by his saying, 'The knower of the unseen, and does not make anyone aware of his unseen except whom He chooses from His messengers (72:26)'. And they are the blessings about which the creations will be asked, because Allah (swt) through them blessed those who obey them from among his friends."

The person asked, "Who are these Hujjaj (proofs)?"

Imam Ali (as) said, "They are Prophet of Allah (saww) and those who succeeded him who are selected by Allah (swt) and they are the Wali-Amr (gaurdians) regarding them Allah (swt) says,

"Obey Allah, and obey the Messenger, and those charged with authority among you." (4:59)

And says, "If they had only referred it to the Messenger, or to those charged with authority among them, the proper investigators would have Tested it from them." (4:83)

The person asked, "What is that Amr?"

Imam Ali (as) said, "That (Amr) is with which the angels descend in the night in which all the wise matters (Amr) are distinguished, from creation (Khalq), livelihood (Rizq) and life and death and the unseen knowledge of the heavens and the earth and the Miracles that is not for any one but Allah and his selected ones and he means between Him and this creations." And they are the face of Allah (swt) regarding which Allah (swt) said, "Where you turn you will find the face of Allah (swt)" (2:115)

[Source: al-Ehtejaaj, Vol.1 PG. 374]