I have to spend the rest of my night proofing my journal article but I want to note that I have a very promising lead for proving that the Secret Gospel of Mark was influential over Alexandrian religious life BEFORE Athanasius. As Telfer notes Athanasius introduced a whole series of reforms to the Alexandria. We don't know what was there before Athanasius but I have noted that Peter of Alexandria and Origen seems to suggest there were perpetual fasts on Wednesday, Friday and - I suspect - Saturday which would align the tradition with Marcionitism.
It was Tim Vivian who drew my attention to the idea that ALL of the Alexandrian Patriarchs up to and including Peter I were Origenists.
At the very least the reader will have to admit that Athanasius does make it clear that a heretical gospel(s) were being employed before his time in Alexandria in the same of 'the saints.' The earliest Festal Letters make this scenario more pronounced but there is a sense that as Athanasius introduced his reforms he did so by aligning them with the heretical traditions which preceded him.
I have only went through five of the forty letters but I think there is enough here to encourage others to make investigations of their own about the presence of LGM 1 lurking in the background of other material from the period.
For those who want to do some investigating on their own here is an online collection of Athanasius' Festal letters.