Is vs.9-20 of Mark 16 the word of God or not? Wayne Grudem says if we are not sure about the content of scripture WE CANNOT BE CONFIDENT of the reformed view of sola scriptura.
As for Mark 16, I suspect that Wayne Grudem did not say things quite the way you've represented him. The canon consists of books; but canonicity is not about individual verses, or even groups of verses. We simply cannot be sure 100% of the time which verses, or words, belong in the text. At least 1% of the New Testament is still in doubt. There are over 300,000 textual variants in the manuscripts of the New Testament; the textual problems that are discussed in the NET Bible are only a small fraction of them. Every English translation includes Mark 16.9-20, even though the editors of virtually all of them would consider these verses not to be authentic. They were most certainly added early on, probably in the third century. They are included for a variety of reasons. First, for many centuries, they were considered to be inspired (although in the earliest period the patristic writers did not so regard them). Second, many today would consider these verses to be inspired. Third, some scholars suspect that Mark ended his Gospel in a way that goes beyond v. 8, but is not identical to vv. 9-20. These scholars would argue that vv. 9-20 were added by a pious scribe to fill in what was lost in Mark's Gospel. So, although they might not be considered scripture, they would still be the text that has been considered from very early on to be a part of Mark's Gospel. In the last few decades, the scholarly opinion has largely turned: now, most scholars would argue that Mark intended to end his Gospel at 16.8 and that the 12 verses added afterward were done by a scribe who misunderstood Mark's literary intention. This is the view that I would adopt, but I am not absolutely sure of it. Recently, some work has countered this, arguing for the older view that the real ending of the Gospel is lost. Is it safe for us to toss out a passage that has had a place in the church for centuries just because we're pretty sure of our own theory? Finally, there is also a tradition of timidity regarding this text. Frankly, editors of Bibles put these verses in often simply because they are afraid that their Bibles won't get read if they don't. The early version of the New English Bible was bold enough to exclude these verses, along with John 7.53-8.11 (which has an even worse pedigree though carries more emotional baggage with it for most Christians). But the Bible didn't sell well, so the editors put the verses back in. The NET does something that is unique I believe: we offset these verses and put them in smaller font. That makes them harder to read from the pulpit! And we also have a lengthy discussion on their inauthenticity.
All in all, we simply cannot be sure 100% of the time what the original text of the Bible said. But this does not impact inerrancy or scripture's sufficiency. Take a look at my essay on "Inspiration, Preservation, and New Testament Textual Criticism" (or some such title), posted on netbible.org.
And remember: even though we are not completely sure of what the original said, based on an examination of the thousands of textual variants, this much we can say: No cardinal doctrine is impacted by any viable variants.