OK I just loaded ToB (after finishing SoA) and had the following question...

If I go ahead and reload my "Final Save", beat Irenicus again and then get sent right into ToB, will Watcher's Keep be an available area in the new setting of ToB?

Or will it exist in the Athkatla area and then be inaccessible after I defeat Irenicus, meaning that I have to reload a save game from Ch 6 to do Watcher's Keep?

Hope my question makes sense and thanks in advance for any help.

Watcher's Keep shows up on both SoA and ToB area maps, i.e. it's accessible from both, so no worries.

So it's not one map or the other? I was under the other impression after reading the manual. But thanks for the clarification.

So I guess the best course of action is to reload "Final Save", beat Irenicus again and go from there...

No, I said both... Once you beat Irenicus you get a new world map, and Watcher's Keep shows there. But it also shows up on the old one if you aren't that far yet when you install ToB.