The early Christians, including Paul and John, did not see themselves as departing from monotheism but as expanding the understanding of God's nature to include the incarnate Word, Jesus Christ, as fully God.
I agree. That was also true of Philo, Justin and the writer of the Ascension of Isaiah. They were adamant that Logos (or any of the names used) was an emanation of the Most High not a God apart or rival God.
Justin as a trained philosopher like Philo understood the provenance of these concepts, he had no problem with that, why do you? I think it is interesting his choice to use of Socratic dialogue, a literary mouthpiece (Trypho) to deliver his doctrinal thesis.
Since you expanded a smidge on the topic, I'll throw the door open and say the so-called Gnostic branches of what became called 'Christianity', all regarded themselves as monotheistic. They were attracted to the concept of emanations of The God as explanations for many things including the impermanence of the physical world. Some used passages that equated Yahweh with a second power, this one however did not resist the temptation to seek worship and so created an imperfect world, something not intended by the Most High. They drew from the same OT sources and 2nd Temple traditions as those who limited the emanations to just 2, (Logos and Holy Spirit).
You reference Philo's concept of the Logos as a bedrock for early Christian Christology, suggesting that the belief in a "second power" or intermediary influenced Christian beliefs about Christ.
My wording was unclear, I meant to say that his/their method of interpretation of passages featuring the second power concept was a bedrock upon which later writers like Paul and the writer of John stood.
While it is true that Philo's Logos concept had some influence on early Christian thought, especially in the Gospel of John, the Christian understanding of the Logos differs significantly from Philo's. Philo's Logos is an abstract, intermediary principle through which God interacts with the world, but it is not fully personal or incarnate in the way that Christ is presented in the New Testament....The inclusion of mundane details, such as the names of Jesus' family members (e.g., James, His brother) and his interactions with well-known historical figures, points to the Gospel writers’ intention to root their accounts in historical reality.
You must realize that is circular reasoning. You declare the Gospel to be different from works like the Bacchae because you see the characters and story to be different. I don't. That is the issue in discussion. Since we have touched on the Bacchae, note that Dionysius plays two roles, one as a lowly human and one role off set as a god. The characters all have names and family that are named. Their hometowns (real) and nationalities (real) are included. What identifies the story as a myth/dramatization are the supernatural aspects, Dionysus making it thunder for example.
Is it really outrageous to believe the writer of a play later transcribed as the Gospel Mark, ought to be regarded as myth/dramatization for the same reasons?
Of course a generation or three later were told the story was real and not allegory. That was also true for many lesser educated Greeks, who mistook the allegories as more than that.