Can you play base-game Claustrophobia scenarios with De Profundis included?
Do you have a link for where the designer stated this?

Nobody I know cares about sorting out the cards to switch back to a base-game scenario. The only concern with using De Profundis with the base game is to impose a 3-card hand limit on the Demon player's Event Cards (because the expansion cards are more powerful). You also probably don't want to use the Hellhounds.