Great thread, but I have a couple small issues with it. I hope to explain my point of view here. Sorry, I'm new so I hope I can convey this properly.
1. The Temple Was Sealed
The Temple was seemingly sealed in the timeframe Meetra was on Dromund Kaas.
Delete spaces sorry, am brand new to this.
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The Old Republic CodexHundreds of years ago, the Sith Emperor ordered the construction of the Dark Temple as a burial place for his dead and defeated enemies, "to aid them in becoming one with the Force." Little is known of what rituals the Emperor performed there, but the Dark Temple has become a nexus of powerful dark side energy, and a place where ancient weapons and ancient secrets of the Sith lay sealed away in cavernous chambers. Although the Dark Temple grounds have always been a dangerous place for the weak-willed (the expansion of the Kaas City power grid into tunnels beneath the temple drove a thousand slaves mad), the temple itself remained sealed until recently, when an expedition of power-hungry Sith Lords and their servants breached the gateway. The Dark Council did not sanction this intrusion, nor did it punish the rogue Sith; no one returned from the expedition to punish. Violent electrical storms surrounded the temple for the first week after it was opened, and since then, additional expeditions have been sent by the council to determine the temple’s status. Their reports have not been made public.
We even see this in action when Lord Alaric wants to seal the spirits in so even an attack couldn't rouse the dead spirits awake.
TOR: Buried Power QuestLord Alaric has informed you that a group of Sith have awakened an ancient Sith Lord in the Dark Temple. The ancient lord’s power has driven everyone in the temple insane, and their violence threatens to overrun the entire planet. Lord Alaric wants you to seal the tomb doors by fusing the giant hinges together with a special torch.
You’ve sealed the chamber doors in the Dark Temple. Report back to Lord Alaric at Outpost Warden.
It's unlikely that Vitiate let the spirits remain powerful for centuries after capturing them:
Lord Alaric"A thousand years ago our own emperor buried his enemies here. These enemies remain powerful, even in death - which is why the temple was sealed."
Kel'eth Ur as well seemed surprised that he had been trapped for almost a millennia, and seems to imply that this is the first time he's seen another being since then.
Kel'eth UrMy internal chronometer tells me it has been exact 963 years since this holocron was made. Have the sith truly lived in fear of me for so long?
Fortunately, I had just enough time to make this - an interactive image that would transmit my secrets to anyone that was bold enough to find my tomb and unlock it.
Kallig - a Tulak Hord ancient being - hasn't seemed to wake up until 300 years after Meetra died.
KalligWhen the weaklings of this planet tresspassed my tomb, I rose, resuming my former life. This temple became my kingdom, and I was once more a Lord of the Sith.
This would point at the temple being closed for 600-700 years before Meetra arrived. Every spirit who was imprisoned seemed to have been there for around 1000 years. If they're roaming around infecting people, it would make sense for him to seal it fairly soon, as opposed to the possibility he waited 700 years and then did it. It doesn't make sense. Vitiate wanted his enemies locked away, he wouldn't wait hundreds of years to finally put a stop to it.
So in summation, it's unlikely the effects of the Temple were leaking out into the city at the time of Meetra's visit. As you've explained in your post, Luke and crew were feeling how bad it was from 2km away. I will divulge details later of how close they got to the nexus.
I forgot about this quote though
Vitiate
Darkly I proclaim, if only I hadn't sealed up that tomb hundreds of years ago, I wouldn't fear an upcoming battle lasting days and nights with this Meetra Surik!
2. The Dark Prophets Used It For Centuries
The Prophets had the benefit of everything that happened in TOR, A warzone in the Light and Darkness War, using it as a religion for their Dark Force, which should make the grounds a lot more potent than it would have been when Meetra was there.
*See attached image at bottom*
Ascension"Such as the fact that this world was a colony of the Sith Empire, but faded into obscurity until after the Great Hyperspace War. It was rediscovered by a Sith armada wandering for two decades. Sound familiar?"
A muscle in Vestara's jaw clenched, but she remained silent. Ben's agitation in the Force increased.
Luke continued. "They rebuilt this world. It became the capital of the next Sith Empire. Until that fell. Its next round of rediscovery came by someone who rejected the Rule of Two, who thought that there was more than enough of the dark side to go around."
"Dad..." Ben started to say.
Luke held up a hand. "He founded a religion that would become known as the Dark Force. There's an ancient temple there still. It was serviced for centuries by the Dark Prophets. It's so steeped in the dark side that it formed its own nexus - one so powerful it interferes with all weaponry and technology except for lightsabers."
[...]
Their destination lay straight ahead. Home to the Dark Prophets, site of an extremely powerful Dark Force nexus, the Sith temple loomed upward, a black, somber silhouette against the gray daylight sky, rendered mysterious and unclear by the mist that occasionally thickened to drizzle. No lights punctured its cold darkness.
As a result of all the tampering that went on there, technology did not work around the Temple:
Ascension"It's so steeped in the dark side that it formed its own nexus - one so powerful it interferes with all weaponry and technology except for lightsabers."
[...]
Ben could well believe that in its presence blasters and other mundane energy weapons and technology would be rendered useless. The lightsabers were both simpler and more complex; the technology was simpler; but because of their deep heritage with the Force, they were far more complicated than a blaster.
We see this in action in the video game Star Wars Jedi Knight: Mysteries of The Sith; when almost all technology stops working once Mara land on Dromund Kaas.
In TOR however, there are holocrons, spotlights, lights, shields, you can use T7 to fight Vitiate, crates, plasma torches, blasters, etc, that all work within the temple. This indicates the Temple is not nearly as steeped in Darkness as it was in Luke's time.
The nexus was a lot more potent in Luke's time due in part to constant meddling for nearly a millennia.