First, the Dawn of the Artifacts series was written by four different people (#1 and #2 by Jennifer Harding, #3 by Stephen McQuillan and #4 by Aaron Pavao and Mark Dynna). And as the two former are not credited for
Artifacts Unbound, it's uncertain if what each actually planned at the time they were writing differed from what they wrote for the Gamemaster to read in each chapter.
[ Spoiler ]
In Dusk,the PC were actually recruited to go after Mystic Crusader Samriel Lockwood, whom Ehran thinks may have information about the theft of the Sexton of/Sextant of the Worlds (which is wrong), and unexpectedly stumble upon the Piri Reis Map.
In Midnight, they go after the actual thief of the Sext' o' Worlds. Actually, for the entire length of two first chapters, Ehran only appeared in the short intro fiction at the beginning of Dusk, in which he only shows some concern about that artifact falling in the wrong hands, telling Harlequin about it.
In Darkest Hour, it is explained that Ehran tried to get the Phaistos Disc as early as mid 2070 or 2071 but failed because the person he hired crossed him. However, he did not further worry about it before 2072, after the theft of the Sext' o' Worlds, coming to the conclusion that "someone was trying to gather the artifacts (which, if you pay attention, is plain wrong, as there no link between those who stole the Sext' and the ones who stole the Phaistos Disc).
In New Dawn, Ehran is supposed to explain the runners he wants the four artifacts to invoke "magical constructions that this fragile world has not seen for millennia, [guarding] them once and for all against any misguided idiots who would meddle in affairs beyond their understanding."
Artifacts Unbound introduced a new power of the artifacts that did not feature in the DoTA series, making slightly altering events to as to prevent them from being gathered in one place. So AU adventures start with Ehran having no artifact, Aztechnology having the Sext' o' World moved to Bogota, a Chinese Triad the Piri Reis Map, and the Black Lodge the Disc-known-as-the-Phaistos-Disc-while-it-actually-was-in-Visoko and the Shantaya's Compass, with zero practical explanation as to how Ehran lost them and they did end up there beside "The Curse did it."
At the end of AU events, Ghostwalker convince Aden, Hestaby, Aina, Ehran, Harlequin, Lugh Surehand, the Black Lodge Penultimate Master, one Jonathon Reed and a team of runners to gather with the four artifacts along a fifth one, the Shroud of Shadows (you'd notice a number of them necessarily came empty-handed to the party, though some presence only accounts for the fact one adventure leave it up t the PC to decide who get the Shroud). They allow Ghostwalker to perform a ritual that close off the Watergate Rift, stop the shedim invasion and reunite the Spirit of Denver.
You may consider Ehran had been helping from the start Ghostwalker to gather the artifacts. But what was actually written in the DotA adventures makes it unlikely to be more than an afterthough.