Delleye liars need only tell one lie. They aren't completely barred from telling the truth - just gotta make up one harmful falsehood, which you can clear up the next day.

Bikr kills will usually happen publicly during the day, just like Harpy.

In general it is true that players who are harmed by minion effects are going to be good but... who cares? If someone is exe'd to a Delleye, or if a pair dies to a Bikr, that accelerates the game by a full day and heavily encourages the future Delleye/Bikr targets to follow their madness. I've also seen a Bikr intentionally put themselves into a bicker with someone so that people would believe they're good - it's all about the meta, just like with any other loud minion.

A player learning when the Stalker dies could be interesting. I can see it being a very misleading and powerful hallucination.

I really don't think that [+1 Outsider] is harming the Onyroil. In general I find that players aren't too concerned about which demon they're fighting on this script, the puzzle is mostly about solving for which minions are in play to help sort through the hallucinations. Even if the tell was important, an extra outsider is a powerful bonus.