3:16 other writings;  i.e. other writings [as a class] = τας λοιπας γραφας.  The article is probably generic, and refers to all writings.  The untaught and unstable are those seeking to causes division and dissension.  They don't just take Scriptures and twist them, they twist all kinds of writings, in modern times, often Philo, Josephus, or something said in the Mishnah or Talmud.   The word "writings" need not mean scripture alone, and the article in the Greek text need not imply that Peter was officially endorsing Paul as "Scripture" in a kind of offhand oblique way.  It is doubtful if they were thinking in terms of canonical categories at the time.  This is not to say that what Paul wrote was not true.  Clearly Peter regards it as true.  But this text is in no way an official statement of official canonical status.  Without a doubt, Paul's writings later became canonical in a more official sense.

The reason many Christians wanted to interpret this text as saying "Paul is Scripture", is that they want the Writings of the Apostles to take priority over the Torah and Prophets, rather than be dependent on the Torah and Prophets.   They wanted the New Testament to have autonomous authority of its own without regard to the Torah or Prophets.  But since the Apostles themselves considered the Torah and Prophets as the final authority, it was enough for them to report the sayings of Yeshua, and to explain them using the Torah and Prophets.  They were not seeking to establish their own writings as canonical authorities—and then appealing to its canonical authority as the reason they should be believed.  That is circular reasoning.  On the contrary, they always appealed to the Torah and Prophets when they wanted to prove or illustrate an argument.