A great idea to improve the QeA layout is to insert avatar and reply button in the comments and auto insert the user mentioned in the reply comment like Threaded Layout
The idea of the Q&A layout is having one Question and Answers. The comments (replies) belong to the Question or Answer, so they are one level depth replies. There is no discussion threads for the Question and Answers in the Q&A layout. You can see the same logic in Stack Overflow forums: https://stackoverflow.com/questions/10715965/create-a-pandas-dataframe-by-appending-one-row-at-a-time?rq=1
As you can see, there is no way to reply to replies. The comments are considered as one level replies to the Answers or the Question.