I don’t think St Isaac ended his life outside of the Orthodox Church, especially since he taught things like the mutual exchange of properties.
Agreed.
I would also note there are Assyrian theologians who believed in communicatio idiomatum, because remember, their Christology was reformed by Mar Babai the Great along Chalcedonian lines. The Assyrian Church has, in my personal opinion, the aroma of Orthodoxy. I think it was basically the ancient church of the Persian Empire.
Interestingly, unlike with the Oriental Orthodox, it becomes very difficult to argue that there was a schism in 433; the relations between it and the Eastern Orthodox were at times exceedingly close and at other times separated only by geography.
I would argue that the schism with the Assyrian Church actually likely happened around the time when the Assyrian Church, devastated by the genocides initiated against it by Tamerlane, that caused it to become a church of only Assyrians after the Indian portion of it was forcibly appropriated by Rome in the 16th century, was without monasteries and forgot its own canon laws, and began to have a hereditary Patriarchy, where each Catholicos would appoint his nephew as his successor, which violates their ancient canon prohibiting a bishop from designating his own successor. The Assyrians went through a period where they were so devastated they forgot who they were, and in the Europe they were forgotten about, or occasionally written off as Nestorians, particularly by Protestant missionaries who regarded the Assyrian Church (and the Oriental Orthodox, and us, for that matter) as not worthy of existing except as a mission field for conversion. It was actually the Anglo-Catholics who helped them monetarily and by donating vestments and other church supplies starting in the late 19th century who I think set their church on the road to recovery.
I would further argue that we must ensure their recovery includes reunification with us. So many Assyrians were martyred by the Muslim warlord Tamerlane that I feel we owe it to them.