I'm not too sure what this 得 is doing here. If I rewrite as in 2. or 3. is that a) correct and b) still the same meaning?
- 德里克.帕腓特有一个极端得空前绝后的假设,。。。。。
- 德里克.帕腓特有一个极端的,空前绝后的假设,。。。。。
- 德里克.帕腓特有一个极端地空前绝后的假设,。。。。。
Basically, I'm not sure what 空前绝后 is describing. Either 极端 or 假设, or both?
I would translate the phrase as: Derek Parfit presents the extreme(极端) and unprecedented(空前绝后) hypothesis that,
It is not good English to write 'extremely unprecedented'. Either something is unprecedented, or it is not.