If the RCC does at some point in the future accept Union with Orthodoxy on their terms, will we Catholics have to give up all our dogma's, traditions, and devotions which were acquired after the Schism?
Obviously you will have to give up anything inconsistent with the dogmatic tradition of the Orthodox Church. And at that, obviously you will have to give up any post-schism doctrine as dogma that the Orthodox do not care to accept as such. Otherwise, I think you should be fine to keep certain post-Schism doctrinal opinions, traditions, and devotions.
Will we, for instance have to give up the dogma's of the Immaculate Conception
I don't know. Many Orthodox will jump to say that it is a heresy that must be rejected. Some think it is erroneous but it doesn't really matter as it's not fundamental to the salvation story. Many others are willing to allow it as a theological opinion. Ultimately we don't know which party will triumph. Obviously it won't be tolerated as being regarded as dogma.
The Assumption is essentially a common doctrine of the Orthodox Church, that is if you subtract the common Latin opinion that Mary never died. I doubt that the Orthodox will be willing to tolerate that opinion (which isn't even part of the dogmatic definition really). Otherwise I think you're good. It just also won't be tolerated as being dogma.
as well as the post Schism ecumenical councils?
They won't be regarded as dogmatic, representative of the whole Church, or even events of the Church at all. But otherwise their teachings that do not contradict Orthodox dogma should be allowed to be maintained as local traditions of your church.
What about the Marian appartions at Fatima and Lourdes?
Obviously not in the context of Mary appearing to the Church. Otherwise, we won't know until that occurs.
Also what of Purgatory, pardons, statues, indulgences, etc..?
Purgatory as a third state of neither bliss of the righteous or torment of the wicked will most likely have to be rejected.
I don't know about statues. It depends on what is decided to be the theological implications of them.
How much change exactly will the RCC have to undergo in order to be considered acceptable for reunion with Orthodoxy?
Both less than many of the most conservative Orthodox think and more than many of the most liberal Orthodox think.