So, somewhere between the Dawn of the Artifacts adventures and Artifacts Unbound, the items in the Dawn of the Artifacts were used to cause a fluctuation the the Dee Cee rift, right? What happened, and where is it detailed?

Mostly mentioned in Conspiracy Theories, IIRC. Basically, most of the major metaplot NPCs (Harlequin, Frosty, Ehran, a few dragons, y'know, the usual crowd) showed up with the big four artifacts. Then Ghostwalker (I think?) dove back into the rift... which proceeded to close. And explode. With enough power to do a good chunk of damage to the bunker and surrounding real estate.

The closing of the Watergate Rift happens near the end of Artifacts Unbound adventure "A Tale of Two Princes", with the short story in the following chapter, "Praxis", narrating the event itself.

In "Déjà Vu", a Neonet intelligence report on July 12th says Ghostwalker agents have the Phaistoc Disc in Ghostwalker's claws, the Piri Reis Map is in Manchuria, while the Shantaya Compass was last seen in Manhattan and the Sextant of World in Caracas (though that one is probably a timeline mistake). The adventure has the PC searching for the Piri Reis Map first in Manchuria, where Ryumyo aligned Yakuza stole it from the Red Dragon Triad, then searching in Tokyo, and finally bringing it back to Seattle for an auction sale.
In "All-Seeing Eye", around July 18th, the Black Lodge has the Phaistos Disc and Shantaya Compass moved to Washington, while failing to acquire the other two artifacts. The PC must steal the Compass and deliver it to the Draco Foundation. Meanwhile, someone paid a Black Lodge agent to open the warded box that contain the Disc near the Watergate Rift, triggering a mana phenomenon akin to a storm.
In "A Tale of Two Princes", which happens roughly at the same time or very closely after "All-Seing Eye", Saeder-Krupp has the Sextant of Worlds in Tir Tairngire. Jonathon Reed, possibly acting on Nadja Daviar's behalf, send the PC to retrieve it. Their effort is delayed by Tir na nOg government agents stealing it and moving it to Dublin, before they can bring it to Washington, causing the mana storm to worsen.

Ghostwalker and Aina invite Great Dragons Hestaby and Aden, Lugh Surehand, Ehran, Harlequin, Jonathon Reed and the Black Lodge Penultimate Master to gather at the Rift with the artefacts. Ghostwalker brings the Map, Lugh Surehand and Jonathon Reed bring the Sextant of Worlds, the Black Lodge brings the Disc, the Draco Foundation brings the Compass, and Aden brings a mysterious box. Ghostwalker then starts a magical ritual that close the Rift, stop the storm, save the day, and cause a big explosion, killing at least Aina.

How Ehran and the Draco Foundation lost the artifacts after Dawn of the Artifacts, how the Red Dragon Triad got the Map, how Saeder-Krupp got the Sextant, how Ghostwalker got the Disc and how the Black Lodge took it from him, how they got the Compass, is not explained.

So everything the party does in the Artifacts series amounts to nothing?
That sucks.
Anyway, I've got artifacts unbound, and I'll begin reading soon, but wow, that's really lame.

The follow up to DOtA was never completed due to authors no longer being with the company.
The artifacts were intended to start a race for ancient secret caches containing items and secrets from the last age.
( Nuff said)

For me personally artifacts unbound felt more like a reboot then a conclusion.
My home campaign involves using the artifacts to uncover clues leading to a powerful ritual that will seal the rift.
This way I can use both storylines by just replacing the artifacts in unbound with other items or clues.

My rewrite has Ehran working to counter the artifacts's weird repelling magic by giving one to each to certain groups of trusted runners for protection while he works on determining their use. the other artifacts get stolen and it falls to the PCs to track down the rest of the artifacts. The PCs then bounce off a few metaplot characters until Praxis happens.