The Historical Circumstances Which Led to the 'Translatio' of the Relics of the St. Mark to the Venetians

I have struggled with how it was that the Venetian sailors were allowed to enter a church or a martyrium, plunder the most important relics in Egyptian Christianity, manage to load them on to ships and then sail out of Alexandria without being detected by the authorities in 828 CE. The standard Venetian story of the translatio is necessarily obtuse because Europeans were forbidden from carrying on business with the Muslims.

Now from reading Eutychius' Annals at Roger Pearse's site I think I see the circumstances which led to the translatio. I think it has something to do with the revolt of the so-called Emma which Eutychius wrongly (deliberately?) identifies as a Coptic word. He writes:

Then the (supporters of the) Emma (Yma) revolted. Al-’Emma is a coptic word and means "forty." This is why: when the Romans left Egypt, in the time when the Muslims arrived, forty men stayed. In the lower part of the country (=Lower Egypt) they testified, multiplied and continued to do so and were called ‘Y MA, i.e. the descendants of the forty (men). They revolted and paid neither excise nor poll tax. This event was announced to Mamun and he sent his brother al-Mu`tasim, who was a Amir, to Egypt. The Emma fought against him . . .

The continuation of the sentence in Ch. (51, 57.17-18) reads: “and he fought them and killed very many of them. He struck them down and drove out their wives and children and brought them with him to Baghdad.”

I suspect that the etymology of Emma from a Coptic word is undoubtedly deliberate on the part of Eutychius so as to blame the revolt - and perhaps the subsequent loss of the relics - on the Copts. The Coptic histories blame the Melkites for the loss. The footnote to this section provided by Pearse notes:

If the rebels had been descendants of those Romans left, then they would have used a Greek name, not a Coptic one. The letters given however do not permit the Coptic reading of HMA (for forty). Later historians have confounded this revolt with that of the Copts in Basmur, which took place allegedly under Abdel Malek around 750-51. Scholars are therefore divided on the exact date of the last Coptic revolt, therefore. See Sylvestre Chauleur, Histoire des Coptes d’Egyple, Paris 1960, 107 (dating the revolt to 216 AH. = 831 AD); item: C. Detlef, G. Müller. Grundzüge des christlich-islamischen Ägypten, Darmstadt 1969, 146 (both giving around 828-30).