Given Paul's clear - and often stated - self-understanding, there seem to be three simple rules for discerning Paul's audience in his letters:
(1) Unless otherwise noted, Paul writes to a Gentile audience.Whatever Paul says about the Jewish Torah and its obligations - especially the cultural identity markers of circumcision, Sabbath observance, and food regulations (kashrut) - it is significant to note that he (unless otherwise stated) is speaking to a Gentile audience upon whom falls no Torah obligations.
(2) When Paul writes about "Jews," these references are most likely to Christ-believing Jews - including the Jerusalem church and other ethnic Jews - that were full participants in the various mission churches rather than to all Jews in general.
(3) Whenever Paul addresses his Jewish kinsmen (sometimes all ethnic Jews, more often Christ-believing Jews as determined by context), these statements are always the exception - and never the rule - to Paul's normal Gentile audience and these statements are always clearly delineated by direct statements or obvious clues in the text itself.
The question in Paul about Jews and Gentiles together in "one body" is the question of whether the end time ingathering of the Gentiles requires Jewish proselyte conversion (washing, circumcision, Torah observance). Paul answers an emphatic "NO!" to this question. For Paul, "Gentiles as Gentiles" are included in God's "age to come" without Torah observance that never applied to Gentiles in the first place.
Paul's "apparent" repudiation of the Mosaic law - in Romans and Galatians and similar passages - means one thing if directed toward Torah-observant Jews like himself, but it means an entirely different thing if addressed to Gentile converts who as part of God's final, end time action in Christ are now included into the "people of God" - without taking on the specific obligations of Torah observance.