"Mark 15:31 has long impressed me as the strongest, conclusively persuasive textual evidence of an historic Jesus. As Jesus hangs on the cross, Mark here has his enemies cry out,

“He saved [that is, healed] others; he cannot save himself.”


The Greek verbs here are ἔσωσεν and σῶσαι, respectively. Back-translating into Aramaic, one finds his enemies here mocking Jesus’ name."

this is your best evidence? you have no jewish speakers of jewish galilean aramaic who preserve this mockery. you only have assumption. my 10 year old niece said , " how can jesus be a saviour when he couldn't save himself?"my niece has never heard of mark, she simply used her common sense. same thing with the empty tomb, a tomb is not empty because of a ressurection , it is empty because someone must have stolen the christian god. matthew creates jewish conspiracy theory only after he discovers that mark DID NOT KNOW ANYTHING ABOUT guards guarding the tomb.
there is no proof that 15:31 goes back to hostile witness.