Does this DLC pack also work with the two DLC episodes, the Knife and the Witches? Also in the main story do you have to start a new game, or can you access it when you've already started? I'm halfway through my second replay.

I believe that it is irrelevant for the other DLCs. It's a pack of preorder bonuses for the main game after all.

I got all the items from it mid-walkthrough - they just lie in the Corvo's quarters. So, I guess, if you can still visit the pub, you're good.