In English, we use the word prophet, but in Quran two different names are used for prophets: rasul and nabi. What is the difference between rasul and nabi?
A nabi is born nabi, the reason we are provided that He got his first wahi in age of 40 so he got nabuwat in the age of 40, this is not a valid reason. if it is valid then anyone can be nabi.
Milad-ul-Nabi (P.B.U.H) is being celebrated in various parts of Muslim countries every year. Is it proved from Quran and Sunnat-e-Rasool (P.B.U.H)? Does Sahaba celebrated it? If someone don't attends
The answer is no. There is no birthday celebration for the nabi in Islam. Nabi's followers had never done any kind of celebration or birthday party for him when he was alive. Birthday celebration for the nabi is definitely bid'at. Even those who intensely claim that it is not haram accept that it is a bid'at.
About Nabi you should be able to find similar justification, but it largely depends on the culture of the society, in some cultures Nabi is used only for prophets but in other cultures as you have mentioned many might choose this adjective as the name of their children and of course, they do not mean that their children would become a prophet.
halal haram - Is it correct to name the child as 'Nabi' or 'Rasul ...
For example: Ibrahim attempting to kill his son, a nabi praying Allah to flood the entire world, people sleeping in a cave more than three centuries, ... etc. If you are expecting the sea to divide in two, so that Palestinians can escape from the cruelty and the cruel ones would drown, it won't happen.