I want to delete EVERY COMMENT I have ever put here, including questions. Looks impossible, is it?

Under the Terms of Service you have given Yahoo a lifetime right to use your content. Even if you close your account all your content will stay on the data base.

Questions can be deleted up to the point they get an answer, answers can be deleted up to the point a best answer is picked, other than that, things stay.

See https://help.yahoo.com/kb/answers/question-answer-sln4198.html?impressions=true

you can delete comments but you can't delete questions if they have answers

It's impossible. You might be able to remove current comments. You might even be able to remove old comments from questions that haven't made it (or won't be given) the Best Answer part. I don't think it's possible to remove answers after Best Answer was chosen. (I might be wrong.) You can't remove comments if yours was the best answer.

Sorry, but all answers and questions are not yours anymore, they belong to Yahoo. ( but if your answer to a question that has not awarded a Best Answer award you can delete that one)