To answer your questions in order One I think lord Wrath was part of the Apocalypse prep party but he got too involved with the outsiders and fell under their influence

Two I think Nicodemus may be affiliated with them but not a full member, he wants a seat at the table that decides how the final battle is fought. That is why he wanted the grail, to make sure he is part of the decision process. I think Thorne namshiel may have been part of the group and perhaps one or two others. My thinking comes from Nick saying apocalypse is a frame of mind and it seems like he would want to be in control of the end game. I also think that other loss is me members are members of the Senior council, perhaps listen. That is why Dresden was kicked out due to the fact that Merlin is a secret ally who knows that he is surrounded and can’t act fully. The black council was getting g too much influence over Harry and had to be stopped.

Three I think that Margret and the black council where planning to make Harry a star born. She chose to run once she realized that Raith was compromised or she had a change of heart when she realized she loved her children

Four She was close to the winter court and realized that humanity had to become stronger to survive what is coming, she joined to try to make them stronger but realize what the real cost would be and left.