Does Downwell return hidden cards to hand?

No, Downwell does not return hidden cards to your hand.

According to the official ruling:

• Hidden cards are not affected: Hidden cards are facedown cards at a battlefield and are not considered "units" or "gear" while facedown. Therefore, Downwell's effect does not target or return them.
• Cleanup process: When Downwell resolves and returns all units and gear to their owners' hands, the battlefield may become "Uncontrolled" (if no units or gear remain). During the subsequent Cleanup step, any hidden cards at an Uncontrolled battlefield are removed and placed in their owner's Trash.

Because this removal happens during the Cleanup step immediately after Downwell resolves, you do not have an opportunity to play or reveal the hidden cards before they are sent to the Trash.