Meanwhile, derivative like works could still be created as long as they were clear to not confuse consumers.
Look at patent registry
https://www.dpma.de/docs/english/formulare/allg_eng/a9510_1....
Why make it complicated? Just let works fall into the public domain after a couple decades.
If they price they quoted was fair, they should be more than happy to sell it for twice that. If it was too low, well shouldn't have lied. If it is too high, more money to the public.
From the tone of your post, you seem to think this is a problem. If so, why?
If we really care about getting the content released to the commons as early as possible, just apply a single shortened date to everyone.
I don't have a problem with 'buy an extension to copyright', though - that money can fund public goods that benefit everyone.
It is as large an obstacle to the hoarding and disappearing of content corporations do today as any alternative, so I'd go for the easiest, whatever that is.