Neil
We cannot say that the author of Mark "uses his own voice". He writes as an omniscient, impersonal narrator.
That's his voice, as opposed to his version of Jesus' voice, his version of the synagogue goers' collective voice, and his version of a typical "scribe's" voice.
We have no idea what the author thought of what he was writing.
That's a different issue, but as it happens, we agree.
Nor does he "allude" to third party statements
Just in the passage we're discussing,
Mark 1:21-28 (World English Bible, my part-attribution in italics):
Narrator:
21 They went into Capernaum, and immediately on the Sabbath day he entered into the synagogue and taught. 22 They were astonished at his teaching, for he taught them as having authority, and not as the scribes. 23 Immediately there was in their synagogue a man with an unclean spirit,
Explicit third party statement:
and he cried out, 24 saying, “Ha! What do we have to do with you, Jesus, you Nazarene? Have you come to destroy us? I know you who you are: the Holy One of God!”
Narrator continues, quoting his subject in passing:
25 Jesus rebuked him, saying, “Be quiet, and come out of him!” 26 The unclean spirit, convulsing him and crying with a loud voice, came out of him. 27 They were all amazed, so that they questioned among themselves, saying,
Explicit third party statement:
“What is this? A new teaching? For with authority he commands even the unclean spirits, and they obey him!”
Narrator explaining that knowledge diffused widely:
28 The report of him went out immediately everywhere into all the region of Galilee and its surrounding area.
And the real would-be-genre bender, at 15:21 wherein the narrator ominsciently identifies two of Simon of Cyrene's children, and leaves us to ponder "Did the author just suggest that an eyewitness account of the Passion has been passed down within a specific family whose contemporary members he can identify?" Well, did(n't) he suggest that?
He never says that he learned any detail from such and such a source.
Definitely. In fact, in light of 1:21-28, I'd conjecture he thought about it, and consciously decided that the better audience experience could be provided by refraining from that device. We, as audience, are clearly not expected to react to 1:22, "What do you mean? Why would somebody positioning himself as an authority in a short performance improve audience experience?"
(In our earlier discussion, I mentioned length as a factor in decision-making.
Mark is an 11,000-ish word treatment of about 100 characters' reactions to a charismatic vagabond and his young sidekicks; a different length might have gotten a different approach, as might a different focus).
Although, 15:21 suggests Mark's not above playing with the device. At a minumum, 15:21 shows us that he is aware that using the device was available to him (whether he's fact-claiming or fabulating, all the same either way).
That is not history writing.
That's what's uncertain.