Can I rewrite 德里克.帕腓特有一个极端得空前绝后的假设 using 的 or 地 instead of 得？ 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.
For 极端得空前绝后的假设, 空前绝后 is describing 极端, to express the degree of 极端. And 极端 is describing 假设. So 极端得空前绝后的假设 could be parsed as 极端（得空前绝后）的假设, means (unprecedentedly) extreme hypothesis.

And when describing the degree of verb/adj, 得 should be used.

Note that in the 2nd sentence, if you change 得 to 的, the syntax is valid but the meaning changes; it becomes 极端的，空前绝后的假设, both 极端 and 空前绝后 describe 假设, means extreme and unprecedented hypothesis.

For the 3rd sentence, I'll say that using 地 is wrong syntax here.

Here're some references about the differences among 的, 地, 得:
「的、地、得」的用法有何区别？
“的、地、得”的用法