Unofficial Skyrim Special Edition Patch

He should leave after awhile because at that point in the quest his AI routine will return him to the main keep.

The cell is already set as a public area which is the usual solution to trespass issues in places the player is meant to go.

As with the other report on this kind of thing, the ultimate solution is to just wait for him to finish the dialogue. It's a know mechanic the devs intentionally set up to keep people from breaking quests by leaving before an NPC finishes what they were saying. It was a very common problem in the older games to have a quest break because you left a cell before the NPC was done speaking.

Hmmm...very odd that he's complaining about you not being allowed in the Hall of the Dead if it's public. When he complained, I assumed it was somehow still marked as private. Maybe the update mechanic is a bit sluggish, so he thinks it's still private? The obvious solution is, indeed, to wait for him to finish his dialogue...I just jumped the gun. I just thought it was private and that it would be an easy fix to make it public.

The cell is already marked public in the CK. It's not a script issue, and there appears to be nothing we can do to address this. He simply should not be issuing a trespass warning.