Something that is not examined, to my knowledge, is the possibility that at least "Mark" didn't know if Jesus existed really or not. So "Mark" imagined simply the life of Jesus. In this sense, he was euhemerizing Jesus without not even knowing that Jesus existed or not. From this POV, he is not so different from who today (or better: yesterday) wrote books on the "historical Jesus".
How was this Mark's ignorance possible? Between the death of Paul and the community of Mark there was a kind of "Dark Age" of which the same "Mark" was a victim, in terms of real knowledge of the Christian past. So Mark had to imagine first, and then give a meaning to the mere fruit of the his imagination. Something of similar Josephus did, when he merely imagined who was "John the Baptizer" by placing him under the wrong Herod (given the fact that - per Greg Doudna - the true historical John was Hyrcanus II).
So the fact that there is not distinction between history and myth in the earliest Gospel was a deliberate act by "Mark" to give to others definitely the answer to the question if what he wrote was true or not. The others were the his same readers. They decided that what "Mark" wrote was entirely or partially true.
Now, the "Jesus agnosticism" of "Mark" is derived also from the "Jesus agnosticism" of Paul. Also Paul didn't know if Jesus existed or not "in the flesh".
From now on, therefore, we regard no one according to the flesh. Even though we once regarded Christ according to the flesh, we regard him thus no longer
(2 Cor 5:16)
The sense would be: even if Jesus existed really "in the flesh", we don't know and we don't care.
So Paul didn't imagine the life of Jesus, differently from Mark. He limited himself to imagine the meaning of Jesus's "known" act. That meaning was true for him even if Jesus didn't exist really "in the flesh".