There's a great post in the FAQ about your first question: Click me by u/talondearg

In short: Yes, it is pretty much consensus among historians that Jesus existed, even though the evidence seems slim at first.

Towards the second part of your question: Even if we assume Jesus is a fictional character, it would still make sense that we use "AD" and "BC": When counting the years since the birth of Jesus was invented in the sixth century and later adopted in most of the Christian world of the time, everybody believed that Jesus existed. The Birth of Jesus marked a very important event for the western world and nobody doubted it's occurence.

The fact that a significant portion of the population in western countries isn't devout Christian is something that happend rather recently. There was absolutely no reason for anybody to abandon the "AD"/"BC" way of counting years. When people figured out that Jesus probably wasn't actually born in 1 AD, the calender was already so well established that nobody wanted to correct this mistake, especially considering that no one could actually prove a correct year either, in fact we can't to this day.

So it's basically a mix between "People didn't question whether or not Jesus existed or 1 AD was the year of his birth" + force of habit / expense of change.