I think it might be useful to quote Eznik of Kolb, a later Armenian bishop who wrote against the Marcionites as he explains this concept of 'three heavens' above the seven heavens of the creator in some detail. We read:
But first, where did Marcion get the idea of three heavens? It was because Moses said two heavens, but as the sectarians wander in all things, so also in this (point) because one said ten heavens, another seven, Marcion three, and, they want to establish their error in scripture, because these books often speak of 'the heavens' and 'the heaven of heavens.'
When the sectarian are by no one restrained, iquite apart from the holy books the wander; and then, that is why they are at risk in the holy books they take refuge. For we find 'heaven' and 'heaven of heavens' in (our) Scripture. But it is because in the Hebrew language one can not say 'heaven,' as in the Syriac language (it says) no water, or sky, but a plural said.
And there it is evident that by the Septuagint (it was) translated (well) in Greek, they say: 'From the beginning God made heaven and earth,' showing (that question) of a (single) heaven, and in the Syriac language, as we can not say heaven, it says: 'From the beginning God made the heavens and the element earth element.' Although we can not say the singular skies, however, saying hain, that is to say, 'element,' the translation states as part of the sky. In addition, the firmament, which is separated from water, the Septuagint translated heaven where it is obvious that the sky above and the sky are two indoor air, and not three or more." [Eznik Refutation, Marcion 7]
The point of course is that Clement goes into so much detail with respect to the seven περιβόλων which surrounded the desert tabernacle that we can be certain that he is speaking about the Church of St. Mark in similar terms (i.e. that seven περιβόλων literally 'enveloped' the structure). One might even wonder if the Church of St. Mark was a replica desert tabernacle. This would certainly have interesting theological implications especially after the destruction of the temple. It would also provide a context for why baptism was called ἀπολύτρωσις (= גאלה).
The point of course is that Morton Smith misread and mistranslated the Letter to Theodore. The perplexing passage in Stromata 5.6 which mentions the 'seven περιβόλων' is clearly the key to unlock the reference to the Church of St. Mark in the text discovered at Mar Saba. What Smith translates as:
[St. Mark] brought in certain sayings of which he knew the interpretation would, as a mystagogue, lead the hearers into the innermost sanctuary of that truth hidden by seven veils
should actually be rendered:
[St. Mark] brought in certain sayings of which he knew the interpretation would, as a mystagogue, lead the hearers into the shrine of that truth enveloped by seven (circuits).
In Stromata 5.6 ἀδύτων certainly means the part of the holy place that circumcised Jews gathered. It is not the 'inner sanctum' or 'Holy of Holies.' There can be no doubt about this. As such what Clement is saying that Christians just by undergoing baptism congregate in the place called ἀδύτων. He makes clear in Stromata 5.6 that this is a unique Egyptian meaning to the word ἀδύτων which presumably was carried over to Egyptian Christianity in to Theodore.
To follow up on Scott Brown's original interpretation of the term 'mystic gospel' I am not sure that the text even holds that the gospel Mark wrote in Alexandria and which the Alexandrian community 'kept safe' was ever kept 'secret' per se. Brown's translation of τοῦ μυστικοῦ εὐαγγελίου alongside my re-evaluation of the physicality of the ἀδύτων based on what appears in Stromata 5.6 puts forward the idea that the 'mystic gospel' was simply the preferred text in Alexandria. There is no reason to believe even for a moment that it was reserved for readings in an 'inner sanctum' at all.
I think it is much more likely that those who became Christians and made their way into the place of worship (ἀδύτων) by crossing the seven περιβόλων outside the Church of St. Mark were symbolically understood to be making their way to the supernal heavens of the Marcionites (i.e. the place where the apostle ascended in 2 Corinthians 12.2. The language of the letter to Theodore also reflects this with its mention of τὰς ἀῤῥήτους διδασκαλίας τῶν Καρποκρατιανῶν at the very beginning (Paul says that he heard 'unspeakable' words).
The point is again that I think the text hasn't been properly understood. Scott Brown's work is only the first step in this regard. We have to think in terms of the physicality of the Church of St. Mark representing some heavenly ascent from the world outside the walls or curtains of the ἀδύτων where all the faithful gathered (= the seven heavens) to the place which symbolized their ultimately 'spiritual home' in the highest heavens.
More work is needed, but this is certainly a step in the right direction ...