: There are a series of terminals throughout Halo 5 that chronicle the journey
: of an 'undesignated Forerunner Builder'. He is searching for someone or
: something called 'Bastion', and unwittingly finds himself on Genesis
: during the events of the game. The final terminal details that the domain
: is 'repairing' (either him or itself, or perhaps Bastion?), whilst the
: ancilla and the Warden are distracted... and then he finds Bastion,
: 'alive'.

: So, who or what is Bastion? I'm guessing it's a place, as the way he talks
: makes it sounds like a last defence... a refuge of some kind. These
: terminals prove that there are other Forerunners alive out there, so
: perhaps he is searching for their final enclave. It's worth noting that, I
: believe, the terminals span the hundred thousand years since the rings'
: activation, as he talks about both the Domain and the Organon being
: destroyed after the firing of the Halo array.

Those terminals are perhaps the most inscrutable hints in the fiction I think we've ever gotten, which is both intriguing and frustrating.

As you say, depending on how you read the different lines Bastion could be a person or a thing, and whether you start from one interpretation or the other your comprehension of the rest of it completely changes. Like this:

"Bastion's location is confirmed. How, though? After dispersal of the Halos, an impossible act of reconciliation would be required. It may still be possible."

Is the impossible act of reconciliation the location of Bastion? Or the ability for the speaker to make it there?

With that said, I lean towards Bastion being a place, mostly because the clearest indicator of what noun it could be seems to me this one: Bastion is absent. Not destroyed; simply missing. No other facility is near enough. A new course of action is required.

It's still possible that the second and third clauses are independent thoughts, but they read as Bastion is absent, and no other facility (like Bastion) is near. Like you bring up the stuff about Bastion being "alive" seems to contradict that, but ¯\_(ツ)_/¯

I believe the basic timeline you elucidate is correct. It starts in the final days of the Forerunner-Flood War, and then basically extends to the events of Halo 5. It makes sense that in the final days not all the Forerunners fled to the Greater/Lesser Arks, or could even flee there, so other rally points across the galaxy within the sphere or out of it might have been established (I still believe that somewhere there should have been some Forerunners who actually used the shield worlds as intended.) To me it reads as the Builder ultimately wound up at Genesis, either by being composed or some other transference. Cortana sees him as a threat and tries to take him out, but as the Domain reopens he's able to figure out where Bastion is—as possibility more living Forerunners.

One of the most interesting tidbits is the question the Builder has about how the Warden "can walk once more", and there's an implication either that he was tied to the Domain somehow (aka 'how can he be alive') or that he was somehow imprisoned (aka 'who could have released him'?)