Yes all circumstanced would be different. Impossible to tell how it would have played out. My opinion is that the historical Jesus would not have been born. Christians may think otherwise.
The Bible says in Galatians 4:4, that "when the fullness of time had come, God sent His Son." Thus, when Jesus was born and what was going on was not happenstance. The point in history and the circumstances of when it happened (for example, the Romans being in power, and their use of crucifixion on wooden crosses) were foundational to Him accomplishing the work the Father sent Him to complete. He had to be raised up on a tree at His death in order to take the curse of the sin of His people upon Himself.
The earliest accounts of Jesus were written only one generation after Jesus's death. Plenty of first-hand accounts were still available. There are many of corroborative accounts of Jesus and other historical accounts of this period.
Christianity had spread to Greece in 50 AD, only 20 years after Jesus's supposed death. In an age where information moved so slowly, it's remarkable that Christianity had spread so quickly.
It's fair to dispute the details of the canonical/apocryphal stories—surely those are contrived—but the idea of Jesus had to come from somewhere. No one else was interested in taking the credit, which should tell us something.
What present-day meme will reach a similar level of distribution and hazy origins? Will we see future generations speculating about the true nature of Harry Potter, or the Jedi, or Chuck Norris's superpowers, or the one true Nyancat?
In short, it's highly unlikely Jesus is purely fictional, and there was almost certainly someone known as Jesus executed by Pontius Pilate. Is this going to make a difference to your beliefs in any direction? No.
Those writings are doing a lot of heavy lifting, since that's about all apologists can hold up as 'contemporary corroboration'.
Antiquities was written in the last decade of the first century, so at the further end of your 50-60y range. As you note, the wording in the Jamesian reference is not very compelling in itself, and historicists generally argue that Josephus would have explained his terminology much more carefully than we see in that section, so there's some reason to doubt it was in the original.
The longer section from that work that apologists refer to is clearly an interpolation. The style is completely wrong for both the author and the context. The previous pages are describing taxes, protests, massacres, etc and then there's this very brief and spectacularly flowery prose that attests to Jesus being the Christ, and to the ten thousand wonders he did, and then on the next page the author continues with 'And another terrible thing that happened to the Jews ...' -- which only makes sense once you remove the clearly forged / added page.
At which point we're really short - a rounding error away from zero - on any corroborating stories from the 1st century.
Yes, Mithras, Dionysus, Zarathustra, and other pantheonic cults have quite a bit of similarity to the messianic figure at the center of Christianity as well as its early rites and rituals. Whether Greek, Iranian, Phoenician/Hebraic fusion doesn't really change the convergence that arose as a consequence of the Roman policy on toleration of local cultures.
Mithrasism was a good candidate to replace early Christianity, as they were rival religions. Christianity persecuted Mithrasism starting in the fourth century CE, all but wiping it out.
Information would move quite fast within the Roman empire, especially in the Mediterranean basin. There was a neverending circular movement of goods and people there. Soldiers, mariners, businesspeople, bureaucrats, learned men. Getting from Roman Palestine to Greece by ship was probably faster than getting from Roman Palestine to interior Syria by caravan.
The earliest allegorical story that mentions the character was written around 70 CE - so a good 40 years after the supposed crucifixion.
You assert plenty of first hand accounts were available (they weren't), and that there are many corroborative accounts now (there aren't).
The movement seemed to be spreading remarkably slowly until it was leveraged by an opportunistic emperor in the early 4th century. At that time there were dozens of various sects, the Nicene was just another variant, but the orthodoxy that came out of that adoption meant we lost many of the works and details of the others. That rewriting of history would also address your last point, too.
I'm not Jewish, I simply loved the consideration regarding how history might have evolved differently.