ExploringSikhi Ji,
while I am not as learned on Sikhi as the other members here, I still want to add my two cents, more specifically, pertaining to this bit here:
As for this, I am basically asking, why should I believe Guru Nanak when he says that he went to the court of God and got Amrit from God. This is the reason Sikhs believe Guru Nanak to be a figure of authority, right? To me, it seems to contradict with other teachings of Guru Nanak, like God is all-around (how do you get "summoned" when God is omnipresent?), rejection of reincarnation/heaven/hell (why does God even have a court if not to judge you?) and rejection of miracles (sounds superstitious to me).
Like Giyani Ji has already pointed out, if you read Guru Granth Sahib Ji, you should not do it literally. The entire thing is poetry, it is full of metaphors and allusions, it was never meant to be taken at face-value.
I get what you are saying. Being "summoned" to the court of God sounds like you are going to a detached place somewhere far away, and as Sikhi rejects heaven/hell/reincarnation, it makes no sense to you. When faced with situations like this, I tend to go to Guru Granth Sahib Ji and find other places the term is also mentioned, to see if it might be able to shed some light. I did a quick search of "court" in Guru Granth Sahib Ji, and here is an instant where Guru Nanak Ji refers to the "court of the Lord":
ਸਿਰੀਰਾਗੁ ਮਹਲਾ ੧ ॥
सिरीरागु महला १ ॥
Sirīrāg mėhlā 1.
Siree Raag, First Mehl:
ਅਮਲੁ ਗਲੋਲਾ ਕੂੜ ਕਾ ਦਿਤਾ ਦੇਵਣਹਾਰਿ ॥
अमलु गलोला कूड़ का दिता देवणहारि ॥
Amal gal o lā kūṛ kā ḏiṯā ḏevaṇhār.
The Great Giver has given the intoxicating drug of falsehood.
ਮਤੀ ਮਰਣੁ ਵਿਸਾਰਿਆ ਖੁਸੀ ਕੀਤੀ ਦਿਨ ਚਾਰਿ ॥
मती मरणु विसारिआ खुसी कीती दिन चारि ॥
Maṯī maraṇ visāri▫ā kẖusī kīṯī ḏin cẖār.
The people are intoxicated; they have forgotten death, and they have fun for a few days.
ਸਚੁ ਮਿਲਿਆ ਤਿਨ ਸੋਫੀਆ ਰਾਖਣ ਕਉ ਦਰਵਾਰੁ ॥੧॥
सचु मिलिआ तिन सोफीआ राखण कउ दरवारु ॥१॥
Sacẖ mili▫ā ṯin sofī▫ā rākẖaṇ ka▫o ḏarvār. ||1||
Those who do not use intoxicants are true; they dwell in the Court of the Lord. ||1||
ਨਾਨਕ ਸਾਚੇ ਕਉ ਸਚੁ ਜਾਣੁ ॥
नानक साचे कउ सचु जाणु ॥
Nānak sācẖe ka▫o sacẖ jāṇ.
O Nanak, know the True Lord as True.
ਜਿਤੁ ਸੇਵਿਐ ਸੁਖੁ ਪਾਈਐ ਤੇਰੀ ਦਰਗਹ ਚਲੈ ਮਾਣੁ ॥੧॥ ਰਹਾਉ ॥
जितु सेविऐ सुखु पाईऐ तेरी दरगह चलै माणु ॥१॥ रहाउ ॥
Jiṯ sevi▫ai sukẖ pā▫ī▫ai ṯerī ḏargėh cẖalai māṇ. ||1|| rahā▫o.
Serving Him, peace is obtained; you shall go to His Court with honor. ||1||Pause||
ਸਚੁ ਸਰਾ ਗੁੜ ਬਾਹਰਾ ਜਿਸੁ ਵਿਚਿ ਸਚਾ ਨਾਉ ॥
सचु सरा गुड़ बाहरा जिसु विचि सचा नाउ ॥
Sacẖ sarā guṛ bāhrā jis vicẖ sacẖā nā▫o.
The Wine of Truth is not fermented from molasses. The True Name is contained within it.
ਸੁਣਹਿ ਵਖਾਣਹਿ ਜੇਤੜੇ ਹਉ ਤਿਨ ਬਲਿਹਾਰੈ ਜਾਉ ॥
सुणहि वखाणहि जेतड़े हउ तिन बलिहारै जाउ ॥
I am a sacrifice to those who hear and chant the True Name.
ਤਾ ਮਨੁ ਖੀਵਾ ਜਾਣੀਐ ਜਾ ਮਹਲੀ ਪਾਏ ਥਾਉ ॥੨॥
ता मनु खीवा जाणीऐ जा महली पाए थाउ ॥२॥
Only one who obtains a room in the Mansion of the Lord's Presence is deemed to be truly intoxicated. ||2||
ਨਾਉ ਨੀਰੁ ਚੰਗਿਆਈਆ ਸਤੁ ਪਰਮਲੁ ਤਨਿ ਵਾਸੁ ॥
नाउ नीरु चंगिआईआ सतु परमलु तनि वासु ॥
Bathe in the waters of Goodness and apply the scented oil of Truth to your body,
ਤਾ ਮੁਖੁ ਹੋਵੈ ਉਜਲਾ ਲਖ ਦਾਤੀ ਇਕ ਦਾਤਿ ॥
ता मुखु होवै उजला लख दाती इक दाति ॥
and your face shall become radiant. This is the gift of 100,000 gifts.
ਦੂਖ ਤਿਸੈ ਪਹਿ ਆਖੀਅਹਿ ਸੂਖ ਜਿਸੈ ਹੀ ਪਾਸਿ ॥੩॥
दूख तिसै पहि आखीअहि सूख जिसै ही पासि ॥३॥
Tell your troubles to the One who is the Source of all comfort. ||3||
ਸੋ ਕਿਉ ਮਨਹੁ ਵਿਸਾਰੀਐ ਜਾ ਕੇ ਜੀਅ ਪਰਾਣ ॥
सो किउ मनहु विसारीऐ जा के जीअ पराण ॥
How can you forget the One who created your soul, and the praanaa, the breath of life?
ਤਿਸੁ ਵਿਣੁ ਸਭੁ ਅਪਵਿਤ੍ਰੁ ਹੈ ਜੇਤਾ ਪੈਨਣੁ ਖਾਣੁ ॥
तिसु विणु सभु अपवित्रु है जेता पैनणु खाणु ॥
Without Him, all that we wear and eat is impure.
ਹੋਰਿ ਗਲਾਂ ਸਭਿ ਕੂੜੀਆ ਤੁਧੁ ਭਾਵੈ ਪਰਵਾਣੁ ॥੪॥੫॥
होरि गलां सभि कूड़ीआ तुधु भावै परवाणु ॥४॥५॥
Everything else is false. Whatever pleases Your Will is acceptable. ||4||5||
From the above shabad, it is clear that the "court of the Lord" is not somewhere far away, out-of-reach of the ordinary man and woman. The Gurmukhs reside in the court of the Lord. What is a Gurmukh? Someone who has control over the 5 evils, remembers Waheguru/sees the light in all, lives an honest life and serves humanity selflessly. It is clear that Guru Nanak's message was that "the court of the Lord" is within each of us, it is when we overcome our own personal desires and have complete control over ourselves at all times, anyone can enter the court of the Lord irrespective of religion or background, simply put, being in the court of God means having a certain state-of-mind.
Now, growing up, Guru Nanak Ji was exposed to both Islamic and Hindu belief systems. He had friends of diverse backgrounds and was exposed to a multitude of schools of thought. What do I think happened on the day he got Guruship? My understanding is that he most likely had a bunch of ideas floating around his head, he was definitely living an honest life, knew what was important so had control of himself, but had no concrete "belief system". On that day, absorbed in meditation, like Ishna Ji suggests, he probably had his "AHA!" moment, everything fell into place, he was finally able to express his beliefs in the form of the mool mantar, and from that utterance, the rest of Sikhi took form. I don't think that he "spoke" to God, like you said, it makes little sense from a Sikhi perspective, in my mind, it was the day he was finally able to present his ideas and beliefs in verbal form (mool mantar).
To other SPN users, I do think that OP deserves a break, especially about the story of Guru Nanak's "meeting" with God. To be honest, I understand his confusion 100%, I have heard the same thing myself far too many times to count. A lot of Sikhs, when asked, will tell you that Guru Nanak Ji went under water in the river, and didn't come out for three days. That he had gone to meet God, and had come back with a message. Most users on here know to stick to Guru Granth Sahib Ji for that kind of stuff, but a non-Sikh probably doesn't know that (myself included when I started out), when every Sikh around you is telling you the same thing, you'd take it as truth and assume it was from the scriptures. Of course, I have come to realize that most stories about the Guru's have very little scriptural basis. OP, have you heard the one about Guru Nanak going to Mecca? You probably have, it is the most widely-circulated Guru Nanak story out there. He goes to Mecca, points his feet towards the Kaaba, an angry Muslim tells him to move them, Guru Nanak tells the Muslim man to move his feet in the direction where God doesn't exist, the Muslim man points them in the opposite direction, but the Kaaba is still there. No matter which direction the man moved Guru Ji's feet, the Kaaba would be in front of them. Guru Nanak Ji then proceeds to "enlighten" the man about how God doesn't live in any specific place, and the man stands there, amazed at what he is hearing.
The entire story smacks of ignorance. First of all, Muslims do not believe that God resides in the Kaaba. To suggest such a thing is disrespectful of Islam. Secondly, it is a big slap in the face of Guru Nanak Dev Ji. He had been exposed to Muslims his entire life, his most trusted companion was a Muslim man, you'd think he would know the basics of the religion. Furthermore, it is another "miracle" story. Guru Nanak Ji rejected miracles. To have a story of Guru Nanak Ji performing a miracle be perpetuated by his own followers is just sad, he would be rolling over in his grave if he knew.
Here is my advice to you:
1) Do not read Guru Granth Sahib Ji literally. To do so makes you miss the entire message behind it, you won't understand anything if you choose to do so, not only that, it will be downright confusing, a literal interpretation of Guru Granth Sahib Ji has every page contradicting the last. Read it, but keep in mind it is poetry, a lot of things are metaphors/allusions and should be treated as such. If someone quotes something from Guru Granth Sahib Ji and it seems odd, like it goes against what the Gurus would have taught, most likely the person is interpreting it literally. Do not make that mistake.
2) Take any story not featured in Guru Granth Sahib Ji with a pinch of salt. A lot of them have been created (by Sikhs, ironically) to make the Gurus look like road-side magicians. The people who come up with them probably think they are doing good, but it is a huge disservice, not only because it is lies, but because it causes a lot of confusion and doubt among youngsters and anyone in general who may not know much about Sikhi. There may be some authentic stories out there, but it would be a really big headache trying to separate the wheat from the chaff, do yourself a favor and stick to Guru Granth Sahib Ji, there is no lesson in those stories that you wouldn't be able to learn in Guru Granth Sahib Ji anyways.
What I am curious to know though, from the sangat on this site, is how come that story of Guru Nanak Ji is the most widely-circulated one when it comes to how he obtained Guruship? Where did it originate? When? I watched this video a few days ago:
Who made Guru Nanak a Guru? - Question #1 - YouTube watch from 1:28 to 3:33 what do you guys have to say about that? Is it any wonder people are so confused?
I am also curious to know about the "having your deeds read out in the presence of Lord of Dharma" and "being judged according to your deeds", I do not know much about that, if any member could enlighten me as well, I would appreciate it