Oz and Alice did not leave for a hundred years. They sacrificed themselves in order to keep the world from being dragged into the Abyss and died. The way it works in the world of PandoraHearts is that souls travel in golden light for a hundred years after their host has died and then are reincarnated/resurrected (although there are exceptions). Since their souls have been traveling in golden light for a hundred years in the Abyss, it's natural that Oz and Alice would not remember everything. They remember some things, but not quite everything.
I'm not sure how to answer the second part of your question. What do you mean by "real" clothes? Did it appear to you that the clothes they were wearing before were "fake?"