The λογος του θεου (word of God) here is probably referring to the message that was spoken. While at this place in the letter it could possibly be ambiguous, in 13:7 it's difficult to see how the same phrase could be understood as referring to Christ. That said, one of the major themes of Hebrews is the close relationship between God and his word, such that the difference virtually disappears in some places.
In addressing the first issue you bring up, let's look at the immediate context of the statement in 4:12. 4:1 sets the context for this entire section and calls back to 2:1-3 where the writer says, "We must pay more careful attention to what we have heard." In 4:1-2, he now draws attention to the promise of rest that has been spoken. In doing so he makes a comparison to the Israelites under Joshua. They too heard a message, but the author points out it had no value to them.
In contrast, the author wants his hearers to receive the promise of rest with faith, and so he reminds his listeners of what was spoken through David: "Today, if you hear his voice, do not harden your hearts." When in verse 12, then, when the author reminds his listeners that the word of God judges the attitudes of the heart, we see how: for the Israelites, their hearts were judged and shown hard because they did not receive the promise of rest with faith. The entire context, then, of two groups receiving a message points to understanding the phrase "word of God" as the message spoken by the prophets but "in these last days" "by his Son".
Second, you bring up the odd description of the word of God. Hebrews is very unique in the way it treats the Scriptures. In most of the New Testament - especially in Paul's letters - whenever someone quotes the Scriptures, they use "It is written" or some variant. In Hebrews, though, the Scriptures are always referred to as "spoken" in some sense when they are quoted. The word of God is very active/performative in Hebrews: it sustains the world (1:3), it sets up a priesthood (7:21), it will shake the world (12:25-27). It's not surprising, therefore, for the author in this letter to describe God's word as living and active, dividing soul and spirit, judging thoughts and the heart.