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?
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.