Agravaine you are assuming that the assault failed in its goals, but the only bodies Harry & co. find are those of trolls so apparently no one from the assault team died, if no one died in the attempt it makes no sense to withdraw before completing your goal, I think the most likely reason for the attack was as a jail break.
By moving the pieces just so, he* arranged for Michael to accompany a whole lot of WC personnel thru the NN back to Chicago - thus preventing the RCV+ Outsider ambush from becoming a rout & a slaughter.
In PG, Eb himself seems to think that the RCVs combined with the outsiders they encountered would have been much more deadly without a KOC there to deal with 'tentacles and things'.
While I'm certain (from his actions / word choice) that Raschid was offended by what the Merlin was intent on doing to Molly, IMO the real goal was probably to save all those young wardens - and the experienced wizards accompanying them. It was also an effective action against outsiders, which is admittedly something that Raschid has a professional interest in...
So, I'm positing that the Gatekeeper was warning Harry about black magic as part of a goal to save WC lives AND act against outsiders. Saving Molly may have been just a nice bonus...
This seems to fit with Bob's explanation of sending messages back in time, The Gatekeeper couldn't tell Harry what he needed to know to save the WC but he could tell Harry what he needed to know to save molly and as a side effect save the WC.