I think you're a little confused: Othra teaches you the recipe for strong restrain potion, not Bernard; you don't need Bernard back from the swamp, but from the wolves den (you need the dark swamp to meet the witch that allows you to go to the wolves den).
Maybe you haven't accepted the quest from Bernard that allows you to learn restrain potion (More potion), that could be the softlock you're referring to. If that so, after you encounter a green slime in the swamp, complete "preserve sanity" Othra's task and that allows you to do "Lost acolytes" that unlocks the dark swamp.
If you don't want to complete "preserve sanity", the recipe for restrain potion is:
- Ingredient 1. Berry (Red)
- Ingredient 2. Nut.
- Ingredient 3. Nothing
- Ingredient 4. Nut.
- Ingredient 5. Berry (Red)
After you create it for the first time you'll learn restrain potion and you'll be able to free Bernard (after many passages).