"The only reference to "immaculate conception" I could find was the "9th ode the Irmos" of the first week of Great Lent. I was unable to find the actual ode itself, but I am willing to bet that the term "immaculate conception" here refers to the Annunciation, and not Mary's own conception by her parents."
I just looked through the 9th Odes for the first week of Great Lent and couldn't find those words. I wonder if its just a bad, Westernized translation. On the other hand, I seem to remember the term "immaculate conception" from Great Lent though I can't say where it might be.
I looked through the service in English translated from the Slavonic Menaion, and none of the hymns were there that Petrosius mentions (in any recognizable form), except for the one that talks of her as "most pure" -- where the Slavonic is translated simply as "pure," although I don't know what Slavonic word was being translated.
Not that it matters -- we refer to her as "most pure" in many English translations (often for the same word that is translated as "immaculate" by some -- a word meaning "without spot or stain.") We do believe that she was without spot or stain because of her morally guiltless life. At what point the Theotokos reached a state of theosis such that it was impossible for her sin I don't know. We don't speculate on such things. But in general, we consider that one's final state isn't set until the moment of one's death.
Also, with regard to the word "today" in the hymnology that Petrosius emphasizes -- this is an Orthodox hymnological convention. We always speak of a feast in the present tense -- always as "today." So the use of "today" has little meaning in trying to establish what the Orthodox believe happened at the moment of the conception of the Theotokos.