5) Who has knowledge of the past Reckoning?
Some characters are more knowledgeable than others. For each Realm, I think at least one person would have some knowledge of the past Reckoning.
Sousuke Aizen searches for power. I wouldn't be surprised if he found something like this in his search that yielded the Hougyoku, his trump card; how much he knows from the past will be up to Aizen. After all, it is a past event. Did he orchestrate this event of chaos to occur? Somehow I doubt he thought he would lose his Hougyoku. At the moment, he also has Kita, which may or may not add to his repoitoire, if he can get at her memories. I am not sure if Las Noches' own researcher, Syazel Aporrogranz, would recall anything.
In the Real World, Kisuke Urahara is a man of science as well as a man who learns knowledge for its sake. He also created the Hougyoku. If you've been paying attention, Urahara may already be hinting at what he knows from his stores of knowledge. Another possibility for knowledge, if we wish to exploit it, would be the Vizard leader, Shinji Hirako, who seems to be quite knowledgeable in his own right, as well as Ichigo's father, Isshin Kurosaki, IF we want to bring those characters in. Urahara should have enough, the clever hat'n clogs he is.
Finally, there is the Soul Society, where easily the oldest store of knowledge would be. The four oldest members of Soul Society are the Yamamoto, Unohana, Juushiro Ukitake, and Shunsui Kyouraku. Yamamoto (and Unohana) will have their hands full protecting Soul Society. In the manga, when knowledge is lacking, we often see Juushiro and Shunsui deep the archives pouring over old, undervalued records. The search is often started by Ukitake and Kyouraku asks the questions his friend doesn't want to ask.
Everyone will know what is going on after the disaster strikes, after three levels of preliminary madness, when the message is delivered to each Realm. Aizen, as we know, is up to something. Urahara, being the shrewd being he is, has begun to hint at gods' displeasure, but does he suspect a Reckoning as in the 'old' records or a search? Ukitake will disappear into records after receiving a premonition of his own and do some catch up after level two of the preliminary rounds.
The knowledge will give you some idea of what is to come. Most of the knowledge known I am going to leave up to those individuals; I may PM some details as warnings of what's to come, too.
