For example, she was asked, "How many shell casings were found on the roof where the shooter was?". She admitted the FBI had told her the answer but refused to tell the oversight committee.
That bit might be standard procedure: there is an active investigation, and details like that might be things the FBI doesn't want to have made public in case the knowledge of the information might be something they can use.
Now, very, very unlikely that the number of shell casings found will end up being information that is more useful for the investigation if it isn't public knowledge, but if I'm the investigator, I want a blanket embargo on all such information unless it is very specifically cleared for release.
All that said, I don't think if I'd been in charge of security for the event there'd have been someone not in the security detail with a rifle on the roof of a nearby building.
Also, I think if I'd been advising the shooter, he'd have positioned himself just a bit further to his left so that trees obstructed line of sight to both of the security sniper positions but still left him, as far as I can tell, with a clear shot at the podium. He could have just kept firing, probably until he ran out of ammunition.