When words are taken out of context and misinterpreted it causes confusion and that is why all Scripture can be used to help in our understanding of it. Not just reading one verse, but starting at the beginning of a chapter of the book to get the sense of purpose for the writings. The Holy Spirit is the must important part because, as we study the Word he brings understanding to us.
This Bible study, reading should never be done for idle curiosity but prayerfully and meditatively with a heart seeking after God's truth. As an example, I can tell you the Bible says "god is dead". But the verse actually says "A fool in his heart says God is dead." That's a simple way of showing the point of words in context meaning what they say. Another way may be you read something Jesus said that doesn't make sense to you. You could look up the verse where Jesus spoke maybe in another Gospel about the same subject and see if the wording is a little clearer.
As far as what Scripture were meant for which group, etc. we should read it all because there are parts that God is using to speak directly to a person about something but the truth of what is spoken applies to us as well. How many people quote the 23 Psalm when they are in a bad place. It was written by David to the Lord yet anyone who reads it feels like it was meant for them personally. I see it as God's Love Letter to US and he will ensure it is preserved for all Eternity.
Isaiah 40:8 The grass withereth, the flower fadeth: but the word of our God shall stand for ever.
Bzzzzt.
Fail.
Where does Scripture say sola Scriptura?
Oh wait, it doesn't......how ironic: a doctrine which fails its own test.