It is the case that concelebrations are allowed for in the agreement between the Antiochian and Syrian Orthodox Churches.
I cannot say that episcopal concelebrations, I don't believe they have taken place, but they seem to be allowed for.
I would not expect an EO who became OO to renounce the latter councils, but view them in a moderated manner. This would especially apply to those aspects which condemned OO Fathers. It would be proper to view them as important Orthodox documents which should be understood in relation to the universal Orthodox Faith, and not only within the narrow confines of the historical EO historical narrative.
An OO may consider the latter councils important local Byzantine conciliar statements. To not be ecumenical is not the same as being of no value. But an OO would want to read them in their context and understand those aspects which are problematic to the OO Tradition. Constantinople 553, for instance, is almost entirely acceptable as it is, when taken as a document.