Dear Irish Ruthenian,
Yes, we are often at a disadvantage when we debate others on the internet for a variety of reasons, as you know.
In this particular debate, we might want to not use the work of an Orthodox theologian only because the RC Trads will say that we have been "infected" by schismatic/heretical views and have otherwise become "invincibly ignorant of the true Catholic faith."
Suffice to say that the Immaculate Conception became a necessity for the West as a result of its acceptance of the Augustinian view of Original Sin. The Cappadocian Fathers, for examnple, see Original Sin in terms of the impact of the personal sin of disobedience of our Forefather Adam, on human nature ever since. So if the Mother of God died, as the liturgical texts for the Dormition clearly state, then she did come under the impact of Original Sin. She never had any sort of sinful "stain" on her soul and was completely sanctified by the Holy Spirit at her Conception (as shown by the celebration of the Conception of St Anne - only feasts of Saints may be celebrated). She was always "All-Holy" and without blemish throughout. For the East, it would have been unthinkable to see the Theotokos as having had a stain of any kind of sin on her soul. This is why the Immaculate Conception dogma is entirely irrelevant for the East. But we can see why it was a necessity for the West with its Augustinian presupposition about Original Sin.
Purgatory is a Western construct intended to define, against Protestantism, that what is not yet completely sanctified, cannot enter Heaven, but is not consigned to hell.
For the East, the final Judgement at the Second Coming of Christ is when we, as composite beings, body and soul, will be consigned to heaven or hell.
Until such time, souls enter a "forecourt" of heaven or a "forecourt" of hell. Those in the latter category can be released from there by the Divine Liturgy and the prayers of the Church here.
St Peter Mohyla of Kyiv did use the word "Purgatory" in his Catechism. The Eastern Patriarchs expunged it from their final "edit" of his Catechism, but Mohyla insisted on using it within his Metropolia. John Meyendorff mentions other Orthodox who understood the doctrines of the Immaculate Conception and Purgatory and who did not hesitate to use it. The Ukrainian Orthodox Saint Dmitri Tuptalenko of Rostov and others even belonged to Immaculate Conception Brotherhoods and took the Western "bloody vow" to defend to the death the IC etc.
So not only Latin Catholics, but Greek Catholics and, historically, individual Orthodox, also hold to the Western definitions on these issues.
Rather than your final statement, you might want to say something like "The Catholic Church commands Eastern Catholics to know and practice their own theological, canonical and ecclesial traditions "nec plus, nec minus, nec aliter. Our traditions are different from those of the West, but no less Catholic. Indeed, our traditions reach all the way back to the original Apostolic Faith of the Catholic Church of Christ that first arose in the East."