In the letter of James, he begins by stating the letter is intended for the 12 tribes of Israel, living abroad. But at the time of writing, was there not only two tribes that existed: Judah and Benjamin? Was northern kingdom, Israel, breeded out to become the Samaritans?
The belief that 10 of the tribes of Israel have "disappeared" or are lost is a myth. All thirteen tribes of Israel continue to exist today as they always have existed. What has been lost to history is the the tribal affiliation for Jews other than those who descend from Levi.
The northern Jewish tribes taken by Assyria were dispersed throughout modern-day Europe. The descendants of these Jews are called the Diaspora, meaning the dispersed. Most retained their Jewish identity and did not intermarry with Gentiles, and these Jewish communities continue to live in Europe even to this day, and though they are Jewish, they do not know their tribal origins. Collectively, they are commonly called Ashkenazi Jews, and these are the Jews James addressed in his letter.
As you say, a few Jews remained in the land after the Assyrian attack and later intermarried to create Samaritans, but these were only a very small number compared to the millions of Jews that went into the Diaspora.
When the temple was destroyed, the Jewish people lost their genealogy records, which prevents a Jew today from tracing their ancestry to a particular Jewish tribe. Only Jews from Levi can be known today, because their family names reflect their tribe (i.e., Levin, Levinstein, Levy, etc.). So all thirteen tribes can be found on earth today, but we do not know tribal affiliations.
