(Sorry for the delay, hope you had a good Thanksgiving!)

They were a lot closer to the subject. Existing scraps are in Greek, the complete text is in Ethiopic. Enoch would of written in the original language (maybe Hebrew or something similar). Those are more strikes against. As I mention at the end, it doesn't read like a Bible book (the book of Mormon at least tries to sound like an OT book...)

The people of the time believed it. The earliest canon lists were compiled because a guy a named Marcion (140 CE) tried to get rid of the OT and reduce the NT. That's only a few generations after the deaths of the apostles. Other "gospels" claiming the names of apostles were not written until about 300 CE (Judas, Thomas, etc.).

The question is, "Protected from what?" I would argue that the Bible needs protection from meddling by the Catholic Church. A politically powerful Church could protect the Bible, but would they protect it from themselves? Given their penchant for doing their idea of "what's best" for people?

No, once is enough. But it does depend on where its mentioned (who were the audience - many OT teachings no longer apply), and whether the teaching is clear (the clear informs us on the less clear).

An easy one is the celibate priest class. The Bible clearly teaches that all Christians are priests. Leadership in the church is in the form of "elders/presbyters" and "bishops" (which are job descriptions for the same group of people). These leaders are to be "one woman" men (either chaste and unmarried, or faithfully married). Also, there is no Biblical support for elders of one church having authority over another group of elders (i.e. the Papacy).

Another is the Mass as a sacrificial ceremony. This gets into more heady theology, but is in danger of violating Hebrew 6:6 (crucifying Jesus again).

There is also the worship ("veneration" ) of Mary. The Bible makes it clear that Mary was a sinner like everyone else. She was the first "Christian", but her "specialness" ends there.

The book of Enoch was not accepted as canon by Jews because it was part of a collection of writings during the Intertestamental(b/w Old and New) period known as the pseudipigrapha (false writings).  Jews believed that authoritative words from God had ceased after Malachi.  That is why none of the pseudipigraphal writings are considered canonical.  It however does not mean they weren't useful because they served as practical daily writings as to how to live as a Jew in their current situation.  They would be like books at a Christian bookstore today.

Enoch was one of several texts that were designated as pseudepigrapha, as another commenter stated. However, the idea that the revelation from God stopped after Malachi is not quite accurate, for two reasons. One, Jews still believe in revelation through Midrash, responsa, and other forms of Torah commentary. But more importantly, that rationale was applied in retropsect in the (off the top of my head) mid 2nd century, when the canon was closed. (though some debate this timeframe)

Actually, a lot of what was tossed in the inter-testamental period was suppressed by the Rabbis (sages) of the period, due to a significant rift in Judaism between the priests and the rabbis. Even before the destruction of the temple (70 C.E.) the rabbis had gained a lot of prominence in Judaism, as not everyone had access to the temple as was required in order to sacrifice. One huge point of debate was over the solar calendar cycles and the tradition of Jewish mysticism known as the merkevah, much of which is closely tied to the book of Enoch and to Enoch himself.

The rabbis, on the other hand, were in favor of a lunar calendar, and did not hold with the mysticism of the priestly tradition. Consequently, many texts that dealt with the merkevah and with Enoch and Metatron were suppressed.

If you want to read more about it, check out The Three Temples by Rachel Elior. She deals with a number of pseudepigraphical works and their histories. It's fascinating stuff!