I will try to keep it as brief as possible. Word Satguru in Gurbani is used for the Sikh Gurus and Waheguru only. No one else can have this status. Guru and Waheguru are in union. Gurbani says that there is no difference between God and Guru.
ਗੁਰੁ ਪਰਮੇਸਰੁ ਏਕੋ ਜਾਣੁ ॥
ਗੁਰ ਪਾਰਬ੍ਰਹਮ ਪਰਮੇਸੁਰ ਅਪਰੰਪਰ ਅਲਖ ਅਭੇਵ ॥
Bhai Gurdas Ji says:
ਗੁਰ ਪਰਮੇਸਰੁ ਇਕੁ ਜਾਣਿ ਗੁਰਮੁਖਿ ਦੂਜਾ ਭਾਉ ਮਿਟਾਇਆ॥ (Vaars)
Being same means being united. Whether Guru and Waheguru are identical, it is dependent on the context. So I would say with certainty that Guru and God are the same meaning they are united. Within Satguru, light of God shines fully. This is why Satguru can unite humans with God as well. Further, Satguru is not a human because Satguru being one with God is free from death and birth. This means Satguru Is not a physical body but the very message that is revealed.
The term messenger in Semitic and Sikh traditions is interpreted differently. Satguru has all the powers granted by God. Satguru is always in union with God and therefore anything He preaches comes to pass as revelation. The communication link is never broken or interrupted. Satguru never speaks out of ignorance or acts against the true path. He is always in tune to Waheguru’s Will. Revelation through him is not passive and he is certainly not serving as a mere tool for the deliverance of the divine message. Guru Sahib says He is speaking as God is directing Him meaning that the source of revelation is God and the message is not diluted or altered in any way. Satguru is perfect and practices the truth whether the revelation has come for humans or not. He never sins, does not hate and is fearless.
Mohammad, for example, was never in direct communion with God nor was he united with Him. This is why in Islam union with God or even His vision is not possible. Islamic God is separate from creation. Even in heaven, the unity does not exist though the souls become eternal. Mohammad allegedly received messages through Gabriel. He made mistakes which were corrected by supposed revelation. A messenger in Islam is someone who receives messages passively. Mohammad always waited for Gabriel to bring the injunctions which means Mohammad remained ignorant of the very religion he was supposed to preach as much as anyone else until Gabriel brought to him the instructions. This means, Mohammad committed sins. Although Islam says their messenger is exempt from punishments but it however implies that he committed sins. Hence, he was not perfect and was a human. He also did not have any special powers granted to him. He again relied on Gabriel to guide him through obstacles. He engaged in anti-Islamic practices until he was corrected or the new injunction was supposedly revealed.
Satguru is more powerful than any other messenger, prophet or avtars who derive their powers and miracles from Satguru. No power, magic or spell can influence Satguru while this is not the case for messengers, prophets and avtars. No one can deceive or cheat Satguru but there are many incidents which prove that Mohammad, Rama Chandra and Krishna were deceived. Satguru has no enemy but messengers, prophets and avtars do. Satguru emancipates all and seeks welfare of all but messengers, prophets and avtars do not. Satguru has control over five vices but others do not. This is a very brief summary of differences. The bottom line is: Guru Nanak Dev Ji is Satguru who is perfect and one with God all the time. From this I hope you can derive something to explain how messenger in Sikh tradition is different than a messenger in Semitic traditions.
When exactly did Guru Nanak Sahib become Satguru is a whole different topic. Gurbani, Vaars and eminent gursikhs believe that Guru Sahib was Satguru from the very beginning (not 1469). Bhai Gurdas Ji says when Guru Sahib came He was a Satguru.
ਸਤਿਗੁਰ ਨਾਨਕ ਪ੍ਰਗਟਿਆ ਮਿਟੀ ਧੁੰਧ ਜਗ ਚਾਨਣ ਹੋਆ॥
Bhatts say:
ਤੂ ਸਤਿਗੁਰੁ ਚਹੁ ਜੁਗੀ ਆਪਿ ਆਪੇ ਪਰਮੇਸਰੁ ॥
This means that Guru Sahib is Satguru throughout all times. Words ਚਹੁ ਜੁਗੀ in general sense refers to entire time span of the creation.
Bhai Veer Singh in Guru Nanak Chamatkaar states that when Waheguru created creation, before it He created Satguru. His logic is that since Waheguru is perfect, His plan of creation must also be perfect. He created suitable environment and necessary elements like air and water for life to thrive on Earth. Therefore, for emancipation of creation, it follows that He must have created or appointed a Satguru prior to creation. In other words, life on Earth requires air, water and plants for survival so God created these before life began on this planet. Since Naam is the only medium for salvation and it cannot be obtained without Satguru, Waheguru appointed/created Satguru before creating the entire creation. This makes His plan perfect. There are so many reasons and proofs to show that at the time of Guru Sahib’s advent He was a Satguru which explains Him revealing Gurbani at a very young age and Him not adopting any Guru or not practicing anti-Gurmat lifestyle. Bhai Randhir Singh states that a Satguru can be appointed by either Waheguru or Satguru. No one can self-appoint himself. There can only be one Satguru at a time and He came to be known as Guru Nanak Sahib on this planet. The same light is spreading Naam in other khands.
I have not discussed sakhi of conversation of Guru Sahib with Waheguru as it requires comprehensive discussion and is beyond the scope of this topic. Pauri Sukhdeep Singh has quoted to prove that Guru Sahib was a bhagat only in the beginning is interpreted differently by scholars. You can read Bhai Veer Singh and Dr. Trilochan Singh’s viewpoint on it. But both do believed that Guru Sahib was already Satguru when the conversation took place. Bhai Randhir Singh too does not state anywhere that Guru Sahib used to be a bhagat only. Again, this is a different topic and requires a lengthy discussion some of which has already taken place. Guru Rakha
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥
