There's no sense, gameplay- and story-wise to make the final point of no return at the start of the Morphic Pool in the Throne of Bhaal. From a story persepctive it's ridiculous since literally every friendly character is gathered at the Watch Citadel at the start of the High Hall. Did your companions simply decide to get wasted at the Elfsong instead while you go after the Netherbrain? Did Owlbear somehow escape? It's truly bizarre.

In fact, I would argue that you should be able to swap characters at any point before going into the first portal of the High Hall (which leads to the battle with the Red Dragon) since you're within quick walking distance to the Watch Citadel. Surely this is an easy and logical thing to implement in a future big patch?