1. Primarily, yes, Paul was referring to the OT Scriptures.
2. Paul didn’t “most likely” read Hebrew. He for sure did. His teacher was Gamaliel, and the language of his instruction would have been Hebrew. His referring to himself as “a Hebrew of the Hebrews,” (Philippians 3:5) almost certainly means this, since he had already identified himself ethnically in the immediately preceding words. I wouldn’t be so quick also to assume that, although the common language of the people was Aramaic, Hebrew was little known and not understood. Among “church goin’ folk” I would assume it was quite well understood. As an aside, I also wouldn’t be so quick, as some are, to assert that Jesus’ disciples didn’t know Greek, and thus had to have written their gospels in Aramaic. Multilingualism in a given population is a very old and very common phenomenon ... except, it would seem, in the USA.
3. That is not how it came across.
4. On this we are in basic agreement. However, it should be noted that there is already pretty clear recognition of NT Scriptures in certain places in other, later NT Scriptures, which I take to be far more authoritative indication of what is canonical Scripture than any earthly one.