In answer to your question about the oldest codex of the Greek New Testament, I believe that the Codex Vaticanus is the oldest, dating to 300-325. Although it was placed in the Vatican Library in the 15th century, claims have been made that it originally may have come from elsewhere in Italy, Alexandria (Egypt), or Caesarea (in Palestine).
My attempt at answering your first question follows.
The Textus Receptus, upon which the King James Bible is based, reads here:
ὥστε οὐκέτι εἶ δοῦλος, ἀλλʼ υἱός· εἰ δὲ υἱός, καὶ κληρονόμος Θεοῦ διὰ Χριστοῦ.
The punctuation reflects that found in the 1881 Scrivener edition. As you have noted, this leads to a reading of
Wherefore thou art no more a servant,
ὥστε οὐκέτι εἶ δοῦλος,
but a son;
and if a son,
εἰ δὲ υἱός,
then an heir of God
καὶ κληρονόμος Θεοῦ
The 1904 Patriarchal Text, the Greek text accepted today by Greek Orthodox Christians, is identical to the above. The majority of Greek manuscripts also agree with the above (less punctuation, which was usually a later addition).
The oldest codices - Sinaiticus (330-360), Alexandrinus (400-440), Vaticanus (300-325) - have a slightly different reading:
ὥστε οὐκέτι εἶ δοῦλος ἀλλὰ υἱός· εἰ δὲ υἱός, καὶ κληρονόμος διὰ θεοῦ.
where the final clause reads heir through God (κληρονόμος διὰ θεοῦ) instead of heir of God through Christ (κληρονόμος Θεοῦ διὰ Χριστοῦ). This reading is also found in Papyrus 𝔓46, which dates to the late 1st or early 2nd century. This is the reading upon which the NASB is based.
In addition, however, there are seven (7) other variations in the Greek manuscripts we have:
- ... καὶ κληρονόμος Θεοῦ - "and [an] heir of God"
- ... καὶ κληρονόμος διὰ θεόν - "and [an] heir on account of God"
- ... καὶ κληρονόμος διὰ Χριστοῦ - "and [an] heir through Christ"
- ... καὶ κληρονόμος διὰ Ἰησοῦ Χριστοῦ - "and [an] heir through Jesus Christ"
- ... καὶ κληρονόμος θεοῦ διὰ Ἰησοῦ Χριστοῦ - "and [an] heir of God through Jesus Christ"
- ... καὶ κληρονόμος διὰ θεοῦ ἐν Χριστῷ Ἰησοῦ - "and [an] heir through God in Christ Jesus"
- ... καὶ κληρονόμος μὲν θεοῦ συγκληρονόμος δὲ Χριστοῦ - "and [an] heir of God, and fellow heir with Christ"
The editors of the Nestle-Aland Critical Greek Text sift through all of these variants and make a call as to which reading they believe is most "certain". The NASB is based on the 26th edition of the Nestle-Aland Greek Text. In this case, the Nestle-Aland editors opted for the verse as found in the aforementioned codices.
A discussion of the above can be found in Bruce Metzger's Textual Commentary on the Greek New Testament