Not all Jewish dispersion was the result of conquest or persecution. Plenty was nothing more than the search for opportunity elsewhere.
At the time James wrote the largest ‘Jewish’ city was probably Alexandria, Egypt. The large established Jewish colony there is why we have the Greek Septuagint.
At the time James wrote (58-60 AD) Jerusalem was still the commercial hub of the eastern Mediterranean.
Precisely. A large colony of Jews fled to Egypt at the time of the Babylonian conquest. Much later, at the time of James’ writing, Nero was persecuting and scattering Jewish Christians and their converts. Persecution and dispersion is nothing new. We still see it today.
My point is that the 12 tribes dispersion mentioned by James included Israel, Judah, and the ingrafted Gentiles. All who believe.