Teleportation in Merchants Guild not lifted if Schwarzzahn is transformed into a chaos being
If Schwarzzahn is corrupted into a writhing mass of primal chaos, and subsequently killed, the teleportation restriction is not lifted from the level and with no apparent way to lift it. Also, the WMoPC is not named, as is typical to other named monster being transformed, which might explain that the game doesn't recognize him being killed.
Tested with a pet balor who did the trick. Simply killing Schwarzzahn with a companion does make the level behave normally.