It is plausible that the death of the Gargoyle and Covenant of the Four Thieves occured before the Breaking; but that would mean Jacko and the Order of Thieves at least as old as Wolfgang and the Empire of Akalabeth.
Well, we know that the Covenant was formed in the Underworld. There used to be an entrance to the Underworld from Hythloth, which one can presume was sealed either during the Breaking or before the breaking...
...but then, is it Prideful of us to presume that just because we never found an entrance to the Underworld from Hythloth after the Breaking that there wasn't one? The fall of the Shadowlords opened a new chasm nearby which gives entrance, but I wonder if there couldn't have been one still in Hythloth. That is, after all, how Beh-Lem came through at one time.
As for the second theory, stranger things have happened. How old is the Widow?
Or then again, Jacko doesn't sound very... mm... stable. My friend's grandmother was pretty sure she'd met Lucille Ball, but it turns out the facts which led to that memory belonged to the woman's sister.