Whoever...is correct. "Who" is a question word, and asks a question. "Whoever" clearly doesn't ask a question. If your not interested to the answer to the question "Who did it?", then you say: Who did it is not important. This is a hidden question. Whoever did it was not amateur. This one is describing the person who did it. We are not asking who did. Nor is there a hidden question. Yes, it is very advanced, and caught out two natives. Good luck with your exercises.

In both cases, it is better to say "whoever" instead of "who". Usually, when a sentence starts with "who", it is a question. Question: "Who did this?" Answer: "Whoever did it was not an amateur."

조중현, The commonly accepted definition of "whoever" is "no matter who" ; "regardless of who". Whoever did it was not an amateur. = No matter who did it, that person was not an amatuer. Who did it was not important. = It was not important who did it. The adjective "important" is not used in formal English to describe a person. He is important. informal He is an important person. formal usage Whoever did it was not an important person. = No matter who did it, that person was not an important person.