Chapter 10:32 - 35: The "Secret" Section of the Gospel of Mark for Theophylactus

The "question of the rich youth" is NOT identified as the "secret" section by Theophylactus. Instead it is the last section BEFORE the "secret" section. Two points of note.  One, Theophylactus apparently is aware of the pagan Emperor Julian's criticism of the previous pericope: 

Whoever has left the relatives whom nature has given him, and, in short, all bodily things for the sake of the Gospel, will receive all these a hundred-fold in the present age and, in the future, eternal life. Will he then also receive a hundred wives? Yes—although the accursed Julian mocked this.

Second, that the "last will be first" ending to the section, introduces the Passion allusion which follows:

And even now, temples have been raised in his name everywhere in the world, which are splendid houses. And what is more, the saints possess all these things amid persecutions—that is what he means by ‘with persecutions,’ that they have suffered persecution and been badly treated and afflicted. Thus those who seem to be last in this world because of afflictions and persecutions will be first because of their hope in God. But the Pharisees, who appear to be first, have become last. Those, however, who, leaving all things, have followed Christ have become first.

With this the rich youth goes away and the "secret" section begins. In his Commentary on Mark, Theophylactus notes: 

Verses 32–34 ... Why does he foretell to them the things that are to happen? In order to prepare the minds of the disciples, so that what they have previously heard they may more easily endure, and that they may not be suddenly struck down in spirit ... And he takes the disciples aside and speaks with them alone. For the Passion was a secret mystery which had to be revealed only to those who were more intimate with him; and therefore on the road he goes before them all, wishing to separate the disciples from the crowd. 

While Theophylactus is unaware of the extra narrative found in the Alexandrian text of Mark according to Clement, he notes that not only is the preceding narrative the beginning of the "secret" section, the narrative which comes after is the effective end of the "secret" section. In the Commentary on Matthew he notes:

But being affected by something human, they persuaded their mother to approach, since they themselves shrank from coming to him openly; for they too came to him secretly, as Mark says. ‘For James and John,’ he says, ‘come to him,’ that is, they come to him privately and apart.”

Interestingly nothing about this "secret" discussion appears in Theophylactus's Commentary on Mark. 

The point here is that without the extra passage in the Alexandrian text, the mystery is that Jesus will die on the Cross. With the extra passage of the "secret" narrative, someone will die in Jesus's place as with the various Islamic apocryphal traditions and most notably Basilides the Alexandrian.