But wasn't the astrolabe a MacPherson back-up plan? I had assumed that H.G. has always known about its existence through that link; but she also knew it was of no use to her due to its limitation. Of course, who's to say H.G. didn't already use it the distant past? What happens if H.G. were to undo her one day? The next season of Warehouse 13 may see a big shake-up on the status quo.
As for the overall arc, I would join in the speculation that Artie himself is the great evil that will be with him for the rest of his days. Each artifact downside usually has a correlation to the origin of the artifact. In the case of the astrolabe, the upside is that you step back across the line into the day before; the downside could relate to the reason you could do that. In effect, we live in a world where two days always exist at the same time; some might even say that makes the world bi-polar.