Textual Criticism and the End of Mark's Gospel: But is it Bible?

I am an Evangelical Christian pastor. I believe that the Bible is God's Word. So do I believe that Mark 16:9-20 are God's Word even though I believe they were not part of Mark's Gospel originally? Here's my (slightly nuanced) answer:

1. Believing that the Bible is God's Word does not imply a belief that there can be no additions or updating to what the original author of the text wrote. For example, I believe that it is more than likely that Moses did not write verses 5-12 of Deuteronomy 34, which describe his death and subsequent legacy. Likewise, there are dozens of places in the Old Testament in which place names have been updated or editorial explanation is supplied (e. g., the comment in 1 Sam. 9:9 that "the prophet of today used to be called a seer"). Thus, I don't believe that the authority of the Scriptures is at stake when I say that verses 9-20 may not have been included in Mark's original Gospel.
2. Canonicity matters. In the history of the canonization of the Scriptures (the process by which the Church identified which books were Scripture and which were not), verses 9-20 were recognized as historic and authentic tradition directly tied to Jesus and the apostles. So even though it is likely that Mark didn't write these lines, they were nevertheless accepted as Scripture at a very early date. So though I'm not completely confident that verses 9-20 should be there, and would advise people against snake handling as part of worship, I'm happy to include them in the Scriptures as the Church has since Justin Martyr's day.