reconciling "Dark Frontier's" plot with the Federation supposedly knowing nothing of the Borg before "Q Who"
Picard and the Enteprise crew knew nothing of the Borg, the federation council and Starfleet command on the other hand were not so ignorant.
If Picard were to look up "Talos Four," would there be a complete file, or just notation that travel to the system is prohibited?
There are probably quite a few things in the Enterprise computer which are walled off, if Riker inquired about the omega particle it is doubtful the computer would tell him anything, but Picard would receive a notification that Riker tried.