I think you could relate the ARK to a strategic command center, like NORAD.
when things get really out of control you put in the key (of doom) aka the index,
Then a command is relayed to stations which carry the heavy weapons, thats nuke silos for us and the fortress worlds (halo is what the covies call it) for the forerunners.
Also the entrance to the micro dyson sphere was in the center of the 'planet', I think the forerunners put it there for a good reason.
Also about the latin of the onyx sentinels, it started speaking latin on its own. Ash made the " i don't understnd what ur sayin" gesture and the sentinel started spakin latin. Or I have got to read ghost of O again.
But the Ark could very well also be an/the exit for the micro dyson sphere(s).
The fortress worlds have a certain time untill they can really fire which would give the reclaimers a certain to get to onyx or another shield world.
One last thing, as the micro dyson spheres are in fact slipspace bubbles which serves to keep the inhabitents safe from the detonations, so would a ship in slipspace be hit by a halo detonation. cus if they were, wouldn't the microdyson be kinda useless?