Not only in questions, but when using "wie" it means that the speaker doesn't know who it refers to. For example:
- Ik weet niet wie dat is - I do not know who that is.
Note: in that sentence, you cannot replace "who" with "that". However, in the original sentence here, you could also say: "He is the one that talks". This can be a good trick to see whether Dutch would use "die" or "wie".
They are syonyms, but diegene has more emphasis. Also see this explanation in Dutch.
Degene doesn't have a trema (diaeresis) btw. Keep in mind that a trema is used only for pronunciation issues (when an apparent combination of vowels should be pronounced as independent vowels), so you will not find it on single vowels (preceded and followed by consonants). This is unlike German, where adding it changes the vowel.