It will probably get ignored (again lol) but here was my theory as far as what happened from the point that Shepard gets hit by the reaper laser towards the end:
The story given to us (from ME1 to ME3) ended up being told similarly to what was told in the bible. No one really knows what happened at the end of the game on the Citadel by the time you see the stargazer and some of the details of what took place have probably been embellished over time, which explains some of the weird stuff that happened when Shepard got to the Citadel at the end of the game (like being able to breathe in open space, the catalyst, etc.) From the point that Shepard beamed himself up to the Citadel to enable the Crucible after he got blasted by the reaper laser and onward, Alliance lost contact with him. There is no possible way that anyone would know what Shepard did, so it makes no sense that the stargazer would know what really happened.
So IMO, nobody really knew what Shepard did on the Citadel. Hackett never made contact with him after Anderson died because Shepard collapsed and seemingly died before that elevator beamed him up to the top of the Citadel. The fact that I've heard numerous complaints that "it stops becoming a Mass Effect story when he reaches the Citadel and is confronted by the Illusive man" gives even more credence to my theory. No one was there that could vouch for what happens after that point of the story. I think the reason the ending becomes so different from what Mass Effect is as a series at that point is because no one really knew what happened to Shepard. From that point on its a story that was made up by the survivors of the war to explain what happened...a myth passed down for seemingly generations. In that context, the ending itself is pretty cool. We basically witnessed the origin of how a new religion was created.
The story had been passed on for so long, by the time we see the stargazers after the credits he's known as simply "The Shepard" like he's a god or something
The fact that the entire series was a story told to a child by his grandfather way farther into the future confirms that for me.
My question is...if you chose the Synthesis ending and destroyed all of the Mass Relays, wouldn't that mean you destroyed almost all life all around the galaxies? If a Mass Relay getting destroyed is the equivalent of a Super Nova, that could destroy a solar system, wouldn't everything including Earth be toast?