Basically my exact thoughts when I finished reading it. If that is your work Quirel, please do continue it.

Planning on reading the main post later and seeing if I have any personal input of worth.

: Feral - they communicate via pheromones and have the instinct to harvest
: enough calcium to establish a viable Gravemind.
: Coordinated - at this point, they are controlled by the Gravemind that was
: created in the first stage.
: Interstellar - the Flood take control of space-faring technology which they
: use to consume the local star system. Subsequently, they spread throughout
: the galaxy to infect more hosts.
: Intergalactic - a theoretical stage in which the Flood utilizes all
: captured technology to depart to uninfected galaxies to further replicate.
: This is further built upon with the knowledge that the Flood originated
: from Precursors who had traveled outside our galaxy.

I know this doesn't directly help with learning what a Gravemind is but it was in the beginning of your post and stood out to me lol.

I think with the short summary of the Flood's origins, in the Milky Way Galaxy at least, that the Forerunner Trilogy provided can't we somewhat conclude that Intergalactic is now plausible but not probable. The Flood having extragalactic origins is a misleading fact if a person is not knowledgeable of the fact that the Flood's origins come from a mysterious powder found aboard extragalactic(presumed to be) cargo ships; and this powder was not identifiable as what is know to the the Flood/Changing Sickness, it simply had the potential to become the Flood. It wasn't until humans, being the curious creatures that humans are, decided to try the powder on their trustworthy pets while oblivious to their actions being the needed catalyst that lead to the appearance of what known as the Flood. Hell even that only happened because small amounts of powder showed positive results absent any signs of Flood-like symptoms so they figured lets keep giving the damn things more magic powder that finally lead to signs of infections.

Basically the Intergalactic stage was information presented years ago, but the novels that Greg Bear penned cleared things up. Think about it, if the Flood could survive a journey between Galaxies wouldn't the whole firing of the Halo rings to starve the Flood be a moot point and just a commendable failure for the Forerunners? Even with the wealth and power of the entire Forerunner Ecumene at their disposal, when the Forerunners wanted to reach another galaxy in an acceptable time frame the result was a single vessel that was considerably small and the crew was 10 or less(not important enough detail to bother flipping through my book). That small vessel, due to the strenuous journey it had to make, still effected slipspace for years(like how the movement of the Halo rings effected slipsplace) and if they'd actually make it to the galaxy was still a gamble. Also this galaxy is the closest possible galaxy.

I know this doesn't help much with the main point of your post, but it was in the beginning of your post and to me stood out as something that has been given clarifying details with recent additions to Flood lore. Plus, bread crumbs man, bread crumbs. To try and comprehend and understand what are these entities we call Graveminds is the same as trying to understand what is the Flood. To me a Gravemind is the Flood and the Flood is a Gravemind. So if we just pool together all facts on the Flood can only lead closer to enlightenment on the subject.