I have read a few reviews of this so know that it takes place at the end of the game but I wasn't sure if that means it is segmented away from the main game/story as its own chapter of if you can visit during the main campaign? Can you jump between areas or are you just in Toussaint all of the time until it is done?

Also, is anything added to the main game? for instance those mutagens, can they be reached from the start of the game or are they only available in the expansion (if, that is, the expansion is as standalone as I get the impression it is), any new armor etc also?

From a few conversations in B&W you could find that the expansion is at the end of the "time frame".
If you do W3 "in order", then both the first/main game and HoS will do a lot to prepare Geralt in many ways to have a more easy life in B&W (from equipment to his level).

The expanded mutagens - you only get this option by solving a specific quest in B&W (as far as I know).
Going back and forth: if you did HoS, and there only the secondary quests (makes sense because of the benefits from B&W), it can be imagined that you can go back from B&W to HoS and back again. Generally you are free to roam every part of the game, over and over again, even if you already finished parts one to three (save off before the endgames, and go from there).