My group is likely to include an ogre and/or a halfling. 

A couple first impressions: 

• I love what I've read so far, in paticular, the optional Backgrounds.
• The opening sections of Part I involve decidedly less life-threatening combat than I've grown accustomed to with this game.  One could definitely begin play with a non-martial character without feeling useless.

How easy would it be to run the campaign with previous editions of the game? My group is keen to play it, but we're not fans of third edition. That's not to say we don't like it, just that we found nothing compelling to make us switch.

Obviously you will have to convert stats etc. of things and find ways of expressing different "effects".  The basic plot and key NPCs and events are all "warhammer".  If you're first edition of course "the world is a bit different" so I can't speak to that, but the 2nd edition take on world fits well enough (timeline is essentially just pre a Storm of Chaos hitting).

I have run 2nd edition stuff converting to 3rd without much difficulty, just some work, so I assume the reverse can be done/

I think it would be close to just as easy to run it with 2 edition as to run it with 3rd. That's basicly because even if it written for 3rd , it is such a huge campaign that each session is bound to require some work to setup and then flesh out and fit into what the characters have been up to. And that part requires a little less work to do with 2nd (imho) than with 3rd.

About the story, i have a hard time imagine that the Players in my groups will not be innovative about trying to figure the cowl out, to a degree that would force the adventure in new directions, and properly use up a likely candidate or two way to early. I properly have to introduce another scapegoat that the cowl can frame and setup, that can sidetrack them for at least 1/3 of the story. Because else it could be hard to keep the suspension up about who the cowl could be.

Confusion about the cowl can early on be that descriptions of cloaked figure in black around during murders/vanishings may make a listener think the figure in black hood that is behind new criminal network.  Later finidng it was skaven, were the skaven using humans?  (instead of the truth, a human using skaven).

You can develop the criminal group more, I'm calling it the Brotherhood, and it can have some minor crime bosses who become part of it.  They all wear black hoods, it's an organization that has a "cell structure" with individual bosses that seldom meet.  Any of these bosses may seem to be the "black cowl".  There may be a "day to day" boss who thinks they are the real power merely taking "advice" from a helpful supporter who only wants a cut of the take (the real black cowl) and is your patsy.  The whole "black cowl" thing can be a red herring - the main villain with the template card can just be the "conspirator" behind the "black dowl patsy".

You can even let them think they have defeated the criminal in Part 1 and Part 2 is carrying them on to clean up the artefact.  This works just fine.

I'm also toying with the idea that the Black Cowl wears is a magical artefact that, among other things, disguises his voice.

And, last but not least, if you're really evil - there isn't just one Black Cowl.  If one falls, the Conspiracy and the Great Conspirator will raise up another.

Another option is to make whichever one you would choose only "think they are the prime mover" and really it's their lieutenant figure in each case who is the real power.

thanks for all the suggestions. There is some really good ones that should be easy to use. Unfortunatly I was not clear enough about my concerns for what my players would be up to.

I am quite sure that they would expect something rotten from at least one of the importants NPC's early one. (they are after all, veterans GM's them self for 20+ years) and what i expect them to do sooner than later is to investigate alot about the 3 major candidates and that could bring them in a situation where their work in "normal circumstances" would be able to rule out at least 1 of them as the cowl. So it's not that i fear they figure out the cowl to early, but more that they can eliminate some of the other candidates too early. To avoid that am consider adding at least a 4th candidate to the pool that has to be figured out first. Because i really think the story would benefit from having many options for the cowl for a long time.

Would they be "waylaid" on the path to early discovery if they found suspects had ills to hide (just lesser ones) on the theory of "it's warhammer, everyone is corrupt", and let them be "users of gray methods for good purposes".  The Conspiracy's principles are in some instances "friendly" as in fewer church trials, more middle class freedom.

Luminary Mauer shelters hedge witches and such from the witch hunters but then discover it's so they can be taken into the College, not so they become sorcerers bwah ha hah (of course perhaps not all of them).

Graf von Kaufman is trying to conceal he is desperate for funds - oh he borrowed money for that expedition and if he can't drum up interest enough to sell of things from it he will have to sell his share in Red Arrow, and he owes the part of this criminal network [a victim!] (of course could be covering up he loaned it to himself and is covering monies with cimrinal income)

Captain Baerfast is intentionally drawing out the succession crisis by supporting Steward Tochfel's endless legal wrangling among contenders (genealogy charts and legal precedents back to the time of the 3 Emperors) - but because he is actually a supporter of the whole Charter concept (which explains how Rambrecht can publish pamphlets full of critiques of nobles and not get arrested) (of course really because councils are easier for Conspriracies to corrupt).