Maggie seems to have been somewhat of a free spirit in the early days and seems to have gotten darker and became a dangerous warlock at some point.  She was a feared witch according to some accounts and even her apparition in the soulgaze between Thomas and Harry called her arrogant.  Regarding the meeting between Eb, Lord Raith, Arianna, and her, it could have happened after the death warrant was established.  Eb implies that he can disregard the Council's decisions when it comes to family in Blood Rites after the raid on Mavra's compound, saying he should have killed Harry but what is the point of being able to disregard the laws and not use it.

He may just may not be able to come to terms that his mother might have been a monster, hence the non reaction.  It was hard enough for him to learn that Eb was the Blackstaff, and at that point in time he didn't know he was his grandfather.

After Ghost Story I think Harry's opinion on monsters is a little different seeing as at "Inez's" insistence he is a monster himself.  He also has resolved the fact that Eb is the Blackstaff of the Council.  Regarding his mother though seems like he wants her to be a good person despite the views of others.