TLDR
- As you follow the hierarchy/chain of command down less details would be known.
- Space Marines likely know a little bit more thanks to the knowledge machines downloading the information into their minds.
- Everyday citizens likely know only basic details.
- A lot of time and elaborate plans went into creating a web of deception across all humanity.
- The galaxy is big. It takes time for any information to travel.
- Knowledge is heavily guarded; including being destroyed.
- Everyday 'Need to know' is really limited to 'Xenos is bad, kill them or die'...
- I would also argue that for the most part no one cares anymore...
Just as an aside:
The Horus Heresy had more than one trigger and had a lot of build up. It was a centuries long plan enacted by the forces of Chaos and even the Heresy itself was a centuries long event. The Emperor made mistakes enacting his own plans and strategies over his many years, but he was still only human... People make mistakes. Nobody's perfect... This question is related to some of the details you mention in your question: Did the Emperor orchestrate the Horus Heresy?
The extent of the deception employed across all humanity's forces combined with the secrecy the loyalists already had in place (gene instability and etc.) created a confusion that, even after the events of the Heresy reached its conclusion, was impossible to fully unravel. While the bulk of the Heresy has eventually been discovered there are details that will always remain hidden (until GW decides expand the current story...).
To my knowledge we never learn of any dissemination of events to the galaxy as a whole following the Heresy. Astropaths likely passed along details, but their communication is not always a back and forth. The details the sending party would deem appropriate would be sent and received some time later... details would have been followed by orders. I would not see many questions asked and fewer answered. Unfortunately, even if there was a widespread publication across the galaxy to everyone explaining all the details of the Heresy who is to say what details could be trusted in such times?
Just as today, details were also held close to the chest and only passed along when necessary. Soldiers (and serfs) don't need all the details. The more information pawns have the more they may turn their attentions away from the task at hand. This is where the idea of 'need to know' comes from. Especially at first information is passed quickly with important details hopefully to be filled in later...
In addition, many details were purposefully destroyed by both sides. The Emperor prior to the Heresy was a practitioner of excommunication which made the idea of information purges prevalent across all humanity. Much information during and following the Heresy was purposefully removed from record. Today there is even a department of the Inquisition tasked with just burning records in case the Imperium's enemies learn of some unknown weakness...
Most of what was passed on/learned was likely 'need to know' and/or embellished details pushing a particular narrative. While such occurrences were common prior, the birth of the Inquisition, Imperial Cult, and etc. all made such practices wide spread. Information is passed on only when it benefits those in charge. Related: What level of education do average citizens have in the Warhammer 40k universe?
All of this combined with the amount of time that has passed makes details that are known in the 41st/42nd Millennium very subject. Hell, even dates in the Imperium can not be trusted! Even more, since the Heresy there have been countless plots and deceptions by Chaos with the attempt to unravel humanity even more.
I have been racking my brain trying to remember the name of the chapter, but a single chapter that I don't even think was special at all was destroyed from the inside by Chaos. The Chapter Master was killed by his childhood friend (his main servant) simply because he was unable to become an aspirant and gain the psychological protections from Chaos that Space Marines gained...
I would also argue that, to many citizens, they don't care. To those citizens that have never even seen a Space Marine or Xenos, and for millennia have tended to their fields and sent away their children to fight on some distant battlefield, they no longer care how it started. They care only that their fields yield a productive harvest.
