Muhammad at his lifetime referred as Nabi in the Quran

God has sent both prophets (Nabiyeen) and messengers (Rusool)

From time to time the divine messages have sent down in the form of scripture as well as in other forms to guide the humanity. Since the Sender of messages is the One, the objective and main theme of all messages are the same to worship the Sender alone but the difference was in languages, description of laws and commands, signs to fit them for the people of the respective community in that era.
[2:38] We said, “Go down therefrom, all of you. When guidance comes to you from Me, those who follow My guidance will have no fear, nor will they grieve.

Revelation of the Quran and Quran’s mathematical proof

The Quran, the last scripture was revealed through prophet Muhammad to the world for all the people that fits all generations, and applies to all eras and societies until the end of the world. But the proof of the Quran was revealed through God’s messenger of covenant Rashad Khalifa after 14 centuries of the revelation of the Quran to bring people back to the scripture and to worship God alone without idolatry.

Quran was revealed through prophet Muhammad
[17:1] Most glorified is the One who took His servant (Muhammad) during the night, from the Sacred Masjid (of Mecca) to the farthest place of prostration, whose surroundings we have blessed, in order to show him some of our signs. He is the Hearer, the Seer.

Quran’s proof was revealed through God’s messenger of covenant Rashad Khalifa
[15:9] Absolutely, we have revealed the reminder, and, absolutely, we will preserve it.
[41:42] No falsehood could enter it, in the past or in the future;* a revelation from a Most Wise, Praiseworthy.
[27:82] When the punishment is almost due against them, we will produce for them a creature from earrth, speaking to them that the people are not certain about our revelations.

[74:1] O you the hidden one.
[74:2] Come out and warn.
[74:4] Purify your garment.
[74:5] Forsake what is wrong.
[74:30] Over it is nineteen.
[74:35] Indeed, this is undoubtedly one of the great (miracles).

[21:2] When a proof comes to them from their Lord, that is new, they listen to it heedlessly.
[72:19] When God’s servant advocated Him alone, almost all of them banded together to oppose him.

Muhammad was the final prophet not the final messenger of God

There are many misunderstandings and erroneous beliefs about prophet Muhammad among traditional Muslims that make them idol worshippers, and idolatry is the only unforgivable sin if maintained until death. If we really believe in only one God and want to get out of this idolatry to save us in the Hereafter, we must know him who he was to show our due respect to him when he is no more among us. He was the last prophet as the last scripture, Quran was revealed through him and was a messenger of God as he was a bearer of God’s message, Quran. God has not only clearly mentioned that he was a human being as well as a messenger of God and the last prophet but also has confirmed that he was the final prophet not the final messenger of God in the following verse of the Quran:
[33:40] Muhammad was not the father of any man among you. He was a messenger of GOD and the final prophet. GOD is fully aware of all things.

All prophets are messengers but all messengers are not prophets

We also need to understand the difference between prophet and messenger of God to remove some of our misunderstandings from our hearts. The Quran has clearly defined prophet and messenger of God that the messengers of God who have been given scriptures are prophets but the messengers have been given other form of messages that can be found in the verses mentioned below.
[3:79] Never would a human being whom God blessed with the scripture and prophethood say to the people, “Idolize me beside God.” Instead, (he would say), “Devote yourselves absolutely to your Lord alone,” according to the scripture you preach and the teachings you learn.
[3:81] GOD took a covenant from the prophets, saying, “I will give you the scripture and wisdom. Afterwards, a messenger will come to confirm all existing scriptures. You shall believe in him and support him.” He said, “Do you agree with this, and pledge to fulfill this covenant?” They said, “We agree.” He said, “You have thus borne witness, and I bear witness along with you.”
[19:30] (The infant spoke and) said, “I am a servant of GOD. He has given me the scripture, and has appointed me a prophet.
[72:27] Only to a messenger that He chooses, does He reveal from the past and the future, specific news.

God has commanded us not to make any distinction between His messengers

God has sent both prophets and messengers to communicate His messages to their people and has repeated commanded us not to make any distinction among them as all of them are honored servants of Him as well as His promise with all of them was the same that if their people follow His message will be saved and reworded.
[2:285] The messenger has believed in what was sent down to him from his Lord, and so did the believers. They believe in GOD, His angels, His scripture, and His messengers: “We make no distinction among any of His messengers.” They say, “We hear, and we obey. Forgive us, our Lord. To You is the ultimate destiny.”
[21:26] Yet, they said, “The Most Gracious has begotten a son!” Glory be to Him. All (messengers) are (His) honored servants.
[3:194] “Our Lord, shower us with the blessings you promised us through Your messengers, and do not forsake us on the Day of Resurrection. You never break a promise.”

Muhammad has consistently been referred to prophet in Quran at his lifetime

There are ways quoted in the Quran how we could respect prophet Muhammad when he was alive and there is also a way how we can respect him when he is no more among us. If we really believe him and respect him, we must follow the message, Quran what he left for us. God has consistently addressed him as prophet when he was alive to communicate with the contemporary believers that may help us respect him properly in his absence and also eliminate any our confusion to misinterpret any verses of the Quran.
[3:161] Even the prophet cannot take more of the spoils of war than he is entitled to. Anyone who takes more than his rightful share will have to account for it on the Day of Resurrection. That is when each soul is paid for whatever it earned, without the least injustice.
[9:73] O you prophet, strive against the disbelievers and the hypocrites, and be stern in dealing with them. Their destiny is Hell; what a miserable abode!
[33:53] O you who believe, do not enter the prophet’s homes unless you are given permission to eat, nor shall you force such an invitation in any manner. If you are invited, you may enter. When you finish eating, you shall leave; do not engage him in lengthy conversations. This used to hurt the prophet, and he was too shy to tell you. But GOD does not shy away from the truth. If you have to ask his wives for something, ask them from behind a barrier. This is purer for your hearts and their hearts. You are not to hurt the messenger of GOD. You shall not marry his wives after him, for this would be a gross offense in the sight of GOD.
[33:56] God and His angels help and support the prophet. O you who believe, you shall help and support him, and regard him as he should be regarded.
[33:59] O Prophet, say to your wives, your daughters, and the wives of the believers to cover themselves of their garments in an appropriate way that they can be recognized and are not harmed. God is Forgiving, Merciful.
[49:2] O you who believe, do not raise your voices above the voice of the prophet, nor shall you shout at him as you shout at each other, lest your works become nullified while you do not perceive.
[66:3] The prophet had trusted some of his wives with a certain statement, then one of them spread it, and GOD let him know about it. He then informed his wife of part of the issue, and disregarded part. She asked him, “Who informed you of this?” He said, “I was informed by the Omniscient, Most Cognizant.”
[66:12] O you prophet, when the believing women (who abandoned the disbelievers) to seek asylum with you pledge to you that they will not set up any idols besides GOD, nor steal, nor commit adultery, nor kill their children, nor fabricate any falsehood, nor disobey your righteous orders, you shall accept their pledge, and pray to GOD to forgive them. GOD is Forgiver, Most Merciful.

Should we commemorate prophet or messenger as we should commemorate God?

I believe most of us will answer to this question “NO” along with a “but” in some of their minds. However, let us try to find the answer to this question in the Statute Book, Quran, who should we commemorate in the following verses:
[33:41] O you who have believed, you shall remember God frequently.
[33:42] You shall glorify Him day and night.

If we can interpret God’s command correctly and can commemorate God day and night, our salvation in the Hereafter has been guaranteed.