First, on the letters of Paul, skeptics like Ehrman make their reputation and their money by taking controversial stands. No one gets his or her PhD by making boring claims like Paul wrote Ephesians. Scholars in general, but skeptical scholars in particular are highly biased toward not accepting the traditional authorship of the biblical books. No one gets famous for saying John wrote John. Even relatively conservative scholars are unduly influenced by controversial claims and tend to cow tow to liberal scholars way too much in my opinion.
It is my opinion that Paul wrote all thirteen letters generally attributed to him. I tend to believe the people who would know, which is the people who knew Paul and the people who knew the people who knew Paul. It was the 100% unanimous opinion of all the churches in the second century that Paul wrote all these books. And they should have known. For example, in AD 150, when Justin Martyr believed that Paul wrote Ephesians there were still people alive who had met Paul. When Polycarp said that Paul wrote Galatians in AD 117 he definitely knew people who had known Paul. He should know whether Paul wrote this book. Most of these books were accepted as canonical because the church leaders believed they had apostolic authority.
I have read very carefully the writing of those who claim that Paul did not write Ephesians or 1,2 Timothy, Titus and Philemon. They argue on things like words used or small details of theology or style of writing. But when I look at these arguments I have always found them to be very unconvincing. Bottom line, they are very weak arguments in my opinion. C. S. Lewis once commented sarcastically (to paraphrase… I cannot remember which of his books contained this thought) that if people were to read his various writings (popular fiction, deep theology, practical Christianity) they would be convinced, based on the arguments used for Ephesians, that at least three or four different people wrote the books that Lewis has written. I agree with him completely.
Based on these reasons, I have concluded that Paul wrote all thirteen books attributed to him. I will have to admit that I am not sure. I will even admit that those who argue the opposite conclusion have at least some evidence to point to. We cannot prove that Paul wrote all these books. That is a fact.
In fact, even these critics will admit that whoever penned Ephesians was at the very least in some sort of Pauline school of thought. Besides, if these books are sufficiently inspired that God caused them to be accepted as part of his inspired Bible, that is good enough for me. The real question is whether these books are inspired, not who wrote them. So, this is not a game-changing argument either way to me, to be honest. If I find out later that Paul did not write Ephesians, it will not shake my faith. But… I believe he wrote all thirteen books for the reasons I just gave.
Ehrman, as I have said many times, is a bona fide scholar. In fact he is a brilliant scholar. But he is extremely biased, he as an axe to grind, and he has an agenda to destroy the faith of believers. Therefore, although his information is generally extremely reliable and he is a careful scholar, his conclusions should be taken with a massive grain of salt. He purposefully says things in the most perjorative terms he possibly can, in order to put the Bible in the worst possible light he can (but always maintaining a high level of scholarship). This is why Ehrman can be very frustrating to deal with.
For example, his use of the word forgery is in several cases an outright falsehood. Even if one or two of the New Testament books turn out to have been written by someone other than what is traditionally accepted, this would not make them forgeries.
For example, the book of Luke is not signed. Yet, Ehrman calls it a forgery. This is rhetoric! Let us say that someone mistakenly attributed the Book of Matthew to Matthew (I do not agree, but that is beside the point), this would not make it a forgery! It is not acceptable to use such a provocative term. A forgery is a work of art produced for the entire purpose of claiming falsely that it is produced by someone else. Even if 2 Peter was not written by Peter, it is possible that someone added the first line of the book, perhaps in the second century. In this case, the book would not be a forgery!!! Just one line would be an interpolation, but the book would not be a forgery. So, what are these supposed forgeries? Ehrman should not make such provocative and simply untrue accusations against the Bible. He does so, not as a scholar, but as a man with an anti-Bible and anti-God bias.
On the third question: Yes!!! Absolutely!!! You can 100% trust that the entire Bible, or at least the original autograph manuscripts of the original Hebrew, Aramaic and Greek texts of the Bible are inspired by God. The evidence for inspiration is overwhelming. My book Reasons for Belief (www.ipibooks.com) is chock full of evidence to support this conclusion. God, in his providence, has produced for us a book written by authors who were “carried along by the Holy Spirit” (quoting 2 Peter 1:21) You can bet your life on the inspiration of the Bible. In fact, you probably already have.