1. You are reinforcing the mundane explanation…
Also, it’s rather presumptive that the case of one rabble-rousing provincial cult leader would be the sole reason for a decree from an empire spanning from Cumbria to Upper Egypt.
2 and 3: They are reporting what the cult believes. Opponents of Scientology reporting that Scientologists believe that L. Ron Hubbard discovered that Space Opera is real and and that all your problems are caused by ancient alien ghosts thrown into a volcano by Xenu using starships that look just like a specific contenporary model of aircraft exept without jet engines is NOT evidence that L. Ron Hubbard actually discovered that Space Opera is real and and that all your problems are caused by ancient alien ghosts thrown into a volcano by Xenu using starships that look just like a specific contenporary model of aircraft exept without jet engines.
Further, Josephus was born a few years after Jesu death, Tacitus two decades. Even in their youth, let alone by the time when they actually wrote their works (post Paul, contemporary with the long-form NT books), Christianity had had more than enough time for accounts to become embellished - giving the entirely unreasonable benefit of the doubt of a cult giving an accurate account of its origins in the first place - and for metaphors to be mistaken for real, especially by outsiders.
Finally, the positivity of Josephus's discussion of Jesus has brought suspicion that the passage is a Christian forgery.
4. Even nowadays, some corpses are never found, while others are found but never identified.
But then, do you have any documentation of said enemies conceding defeat outside the cult's account? If not, we cannot exclude the possibility of the body actually being found, nor indeed no such search taking place in the first place.
People have turned a case of a man getting away with a badly forged passport claiming to come from a country that does not even exist for years of international flights in the far more relaxed times before 9/11 into a spooky Unsolved Mystery™ suggesting alternate timelines or universes.
5. Cults can grow very quickly, and lie about their popularity. Nothing extraordinary here.