Diana Stanley dreams of a sunken city
Diana Stanley's Dark Insight ability does not override the effects of the Mythos environment in play. If it would override the Mythos card, it would state so.

For example, Akachi's ability states:


  Secret Rites - Any Phase: Akachi gains +1 bonus to all skill checks to close or seal a gate. In addition, Akachi may always seal gates, regardless of other game effects.