1) What is the canon? Is it an "authoritative list of books"?
It's better to think of the canon as the list of authoritative books. An official canon may itself be in some sense authoritative, but in that case it is an authoritative list of authoritative books. Even before any "authority" pronounces what the list is, the books have authority and are to be included in the list because of their authority, not because of the authority of the person or group making the list. Thus, a canon list can be wrong.
2) Were the books of Tobit, Judith, Wisdom of ben Sirach, Wisdom of Solomon, and 1 & 2 Maccabees as well as the parts of books such as Baruch, the Epistle of Jeremiah, the Prayer of Azarias, Susanna, Bel and the Dragon, and six additional chapters in Esther, part of the Jewish canon?
No. The evidence is that they were not. That's especially true of the Wisdom of Solomon, which wasn't even written in Hebrew or Aramaic.
3) Were they only excluded in the 2nd century A.D. in reaction to Christian usage of the Old Testament?
a) It's hard to argue that the Samaritans excluded them on that ground. They apparently only viewed the Pentateuch as canonical. Some have asserted that the Saducees agreed with them.
b) Likewise, it's hard to explain the first century Pharisees' exclusion of them on that ground.
c) In fact, what is the supposed documentation? Is it just that they were rejected together with the New Testament books?
4) Was the Old Testament canon closed in the 1st century?
It may not have been closed in the sense of not being open to future additions, but it was closed in the sense of having an identifiable group of already-written books associated with the word "Scripture." When Jesus said, "Search the Scriptures," the Pharisees didn't say "and what books are those?"
5) Objection: "There's no list closing the canon, before Christ."
Even if no such list exists, what difference does that make? The argument seems to presuppose that one needs to have an authoritative list, in order for the canon to have definite shape.
6) Objection: "The three-fold division (in Sirach and in the NT) just refers to three stages of canon development and the 'the writings' category was an open one"
This argument hinges on the assertion that the "writings" category was still open, but where is the documentation to support this assertion? On the contrary, the usage in Sirach and the New Testament seems to point to a fixed body of known works.
7) Objection: "Although the deutero-canonicals are not cited as Scripture in the New Testament there is reliance on them and some of the proto-canonicals are also not cited."
a) This, incidentally, is an undercutting argument with respect to item (3) above, about the reason for Jewish rejection being Christian acceptance.
b) It's just one evidence that the deuteros are not canonical. It's not in itself the definitive proof.
c) There are good reason for including the unquoted protocanonicals.
8) Objection: "Hebrews 11 mentions martyrs of 2 Mac. 7"
Even assuming it does refer to them, there is no particular reason to infer canonicity of an historical account of those people's life.
9) Did Athanasius accept the Deuterocanonical books as inspired? Did he just use the term "canon" differently?
His Festal Letter 39 puts that debate to rest (source):
4. There are, then, of the Old Testament, twenty-two books in number; for, as I have heard, it is handed down that this is the number of the letters among the Hebrews; their respective order and names being as follows. The first is Genesis, then Exodus, next Leviticus, after that Numbers, and then Deuteronomy. Following these there is Joshua, the son of Nun, then Judges, then Ruth. And again, after these four books of Kings, the first and second being reckoned as one book, and so likewise the third and fourth as one book. And again, the first and second of the Chronicles are reckoned as one book. Again Ezra, the first and second are similarly one book. After these there is the book of Psalms, then the Proverbs, next Ecclesiastes, and the Song of Songs. Job follows, then the Prophets, the twelve being reckoned as one book. Then Isaiah, one book, then Jeremiah with Baruch, Lamentations, and the epistle, one book; afterwards, Ezekiel and Daniel, each one book. Thus far constitutes the Old Testament.
5. Again it is not tedious to speak of the [books] of the New Testament. These are, the four Gospels, according to Matthew, Mark, Luke, and John. Afterwards, the Acts of the Apostles and Epistles (called Catholic), seven, viz. of James, one; of Peter, two; of John, three; after these, one of Jude. In addition, there are fourteen Epistles of Paul, written in this order. The first, to the Romans; then two to the Corinthians; after these, to the Galatians; next, to the Ephesians; then to the Philippians; then to the Colossians; after these, two to the Thessalonians, and that to the Hebrews; and again, two to Timothy; one to Titus; and lastly, that to Philemon. And besides, the Revelation of John.
6. These are fountains of salvation, that they who thirst may be satisfied with the living words they contain. In these alone is proclaimed the doctrine of godliness. Let no man add to these, neither let him take ought from these. For concerning these the Lord put to shame the Sadducees, and said, ‘Ye do err, not knowing the Scriptures.’ And He reproved the Jews, saying, ‘Search the Scriptures, for these are they that testify of Me.’
7. But for greater exactness I add this also, writing of necessity; that there are other books besides these not indeed included in the Canon, but appointed by the Fathers to be read by those who newly join us, and who wish for instruction in the word of godliness. The Wisdom of Solomon, and the Wisdom of Sirach, and Esther, and Judith, and Tobit, and that which is called the Teaching of the Apostles, and the Shepherd. But the former, my brethren, are included in the Canon, the latter being [merely] read; nor is there in any place a mention of apocryphal writings. But they are an invention of heretics, who write them when they choose, bestowing upon them their approbation, and assigning to them a date, that so, using them as ancient writings, they may find occasion to lead astray the simple.
Notice that Athanasius treats Esther as Deuterocanonical and that Athanasius thinks that Baruch and the Epistle of Jeremiah were parts of Jeremiah. So, Athanasius' canon is not exactly the same as our canon. It is interesting to note that we see a reckoning of 22 books here. Others in the West have suggested the enumeration was 24 (link to discussion). While the lists don't all agree with one another exactly, the Deuterocanonicals and Esther are the books that tend to be omitted from the list. We have good reasons to keep Esther, but we don't have good reasons to the Deuterocanonicals.