Regarding Margaret, she might have had similar reasons as Listens-to-Wind when he hesitated to tell Harry. It might involve the type of knowledge that can mess you up under the wrong circumstances.

Also, how sure are we that she planned to bear a starborn child? Everything else we know about her suggests her relationship with Malcom was a happy accident, and timing a child's birth can be rather tricky, and it probably wasn't any easier in the 80s or 70s or whenever Harry was born.

It does seem to be the default among the wizard community to let wizards find out the big specific stuff on their own. From his journal, Ebenezer clearly knows a lot about Demonreach (as does the Gatekeeper), but they don't say a word to Harry. There is also Anastasia noting that they don't tell young wizards about the potential for precognitive recognition, Margaret also didn't tell Harry about the guide to the Ways she left with the Leanansidhe, Ebenezer does not seem to have warned Harry that the White Court might have reasons for gunning for him personally, and so on. He knew a lot about the Winter & Summer Fae, but needed an explanation from Bob of the courts' structure. No one ELSE from the White Council needs an explanation of the Denarians or the Swords (Martin, too), but Harry, best friends with one of the Knights, was operating for a while on the assumption that Michael's power might have simply been a self-fulfilling delusion, and had no idea that the Denarians existed until they had been working together for years.

The fairly obvious explanation about the silence over his starborn status is that they don't want him while still under youthful delusions, picking fights with Outsiders before he's ready. Even without the starborn thing, he's been put through the wringer by them a few times. So imagine if he goes into a fight when he's too young and inexperienced, assuming his starborn qualities will save him and he gets handed his ass. That's going to be a huge hit on his confidence, which can impede his abilities to fight them down the road. Whereas the way it works, he's had lots of experience, he faced the Outsiders he did encounter with caution and respect (to the degree Harry can manage that stuff), and so he has more self-assurance in his abilities. Thus, when Ebenezer finally tells him, when it's really necessary, he's receptive and it augments his magic, rather than undermining it.

As far as the Bob question ... I wonder how much Harry really wants to know? I get the impression he was trolling or testing Listens-to-Wind when he asked. Like deep down inside, he'll be fine if they tell him, but other than that, he'd rather put off confirmation that preventing yet another apocalypse is going to be primarily his responsibility.

Anyway, it might be something that Bob doesn't know. He knows about wizardly magic. He doesn't know much about the powers of Heaven & Hell, for example, saying that it's a different frequency of power than the stuff he knows. It would also make senses that the Outsiders and their rules and the powers that affect them would be on an different wavelength as well. And we see from Backup that there are things Bob can be prevailed upon to keep secret from Harry for his own good. Knowledge of the Stars and Stones might be on the list. It could be like balefire, where even the badguys don't want the knowledge spreading, so Justin DuMorne might have told Bob to excise it, or Etienne might have blocked him from learning or revealing it.