Do you think he was referring to Scriptures that are not included in the modern Bible?
2 Peter 3:16-17
There are some things in them hard to understand, which the ignorant and unstable twist to their own destruction, as they do the other scriptures.
Indeed. However, it is also clear that this does not just present itself without authoritative teaching. This is why Jesus had to open the minds of the disciples to the scriptures, and why they had to be explained to the people by the Apostles. If they were self explanatory, all of Israel would have converted. On the contrary, the Pharisees were expecting a different messiah.
Yes.
Indeed, I am sure it seems that way to you.
Which "scriptures’ do you think were being twisted by the ignorant?
I agree, Peter does not say they are impossible to understand. He was quite clear that the Truth had been revealed to the church, and that they could be understand in the context of what Jesus had revealed to the Church.