Looking at your second question first, I would reply that there are a host of matters in scripture which are plain and which any humble reader, though simple and unlearned, may readily understand with the help of the Holy Spirit. There are other matters which are less plain, more obscure, and on which very sincere Christians, evidently used, blessed and owned by God, hold differing opinions.
"The plan of salvation is so clearly taught in the Bible, a child could understand it!" My brother would confidently assert to me whenever we used to get into a theological dispute. (He is a born, raised, and practicing Southern Baptist, as I once was). This indeed sounds logical, and it is something that I used to believe myself. (That is, until I decided to re-read the new testament with a keen eye towards the plan of salvation.) Starting in the Gospel of Matthew, I already began to encounter passages that I thought required further explanation. I found passages that seemed to contradict each other. Eventually, I started to realize something. The plan of salvation that I thought
was so clearly taught in the bible was only taught in specific sections of the new testament, to the exclusion of other verses on the matter which apparently seemed to contradict it! After solemnly refusing to admit that the bible contained contradictions, I began to realize that my church didn't give me the whole picture on salvation. Sure, the "Romans road" made sense (or at least it jibed with my church doctrine), but there is certainly a lot more the bible has to say about the matter of salvation than this select choice of passages!
The more scripture commentary I read from an Orthodox perspective, the more I realize how much meaning, interpretation, and context is hiding underneath the seemingly "bare" words of scripture! I assert that any confidence that we may have regarding a passage of scripture to be "self-explanatory" or "straightforward" is directly attributed to, and subject to: what we have been taught to read into the scripture (from church tradition or commentary), our individual knowledge, bible translation, individual context, etc.
For example, look at one of the most well known verses in the bible, Ephesians 2:8: (KJV)
For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: not of works, lest any man should boast.
Here are just a few questions that a "simple and unlearned" reader could ask of this particular verse: First of all, what is grace? What type of faith attracts grace? For that matter, what IS faith? What exactly is a gift of God: grace, faith, or both? What does it mean to be "saved"? Is this a one time event, or is this something ongoing? Is this something that I can "lose"? Obviously, such an inquisition, however honest it may be, reveals more questions than answers to the individual. Where is this inquirer to turn? His denominational pastor? Why trust him? After all, this person should be able to interpret scripture themselves, otherwise the'yre just relying on "tradition", hence defeating the basic premise of sola scriptura!
I will say that the more I mature in my understanding about the Orthodox faith, the more wisdom and saving knowledge I gain regarding this particular passage and others like it. In fact, during my intial inquiry phase, I thought this verse to be completely opposed to the Orthodox teaching on salvation! (based on my presuppositions and prior "knowledge" of the scriptures and denominational background). Now I see that my initial conclusion was totally biased and unlearned. This just goes to show that anyone can interpret anything into any verse depending upon the particular circumstances they find themselves in, and the knowledge that has been revealed to them.
Finally, I encourage you to read this article on the deficiencies of the premise of "sola scriptura" which may help to illustrate my point.http://www.chnetwork.org/journals/sola/sola5.htm