I like to think about the fact that everyone seems to be in agreement that Nick Valentine's Kellogg moment after the memory den sequence is untapped story.
Are his circuits damaged now? Will he carry Kellogg forever? Is he slowly going to become evil? Does he have multiple personalities now and not even know it?
But what if what he said is true? It's still untapped story. He tells Sole that there are after affects FOR SOLE after the memory den too. What if he never spoke with Kellogg's voice at all and is getting more and more worried about Sole? What if Sole is the one now seeing and hearing Kellogg in random moments with Valentine? Every time they bring it up he gets more and more worried. Sole's brain is the one becoming warped. They're becoming paranoid and unsettled, jumpy and accusatory.
It makes it better if Sole sees what players see and refuses to think of it being all in their head. Sole insisting all of the fan theories and headcanons to Nick but Nick trying to insist that it's all Sole. Both of them worried and trying to get help for the other.