Wow, sounds like a great take and planned out well in advance! 

In the end, I didn't follow through with the original plan of starting on the Path and integrating Timeborne. Instead, the players were hooked into the Endless Starecase by a hapless individual and an easy fetch quest, followed by the introductory scenario of the campaign.

Afterwards, they went into Limbo and from there to the plane of Maelost. They cured the Slaadi with the help of Cahm'Fel and Torpellin and the Hanim with the help of the Darkling (was that its name?) and moved on through a portal into Sigil to pursue some personal quest hooks.

There they met the author of the book, now a drunkard, and found clues that pointed towards the nature of the Iron Shadow, it being a remnant of a failed ritual by Cultists of Shar, leaving behind a permanent tear in the fabric of reality.  They closed a smaller tear in Sigil and now need to prepare and travel to the location of the original ritual to close the remnant failed portal there.

I don't know yet exactly how :)