I currently add this to my qa-config.php
'questions' => '',
Is it possible to add sort by recent answer and comment?
You can't choose the sorting. What you can choose is to show a new section (the All Activity one) as I explained in this other answer http://www.question2answer.org/qa/41479/sort-by-last-comment-and-answer?show=41484#a41484
I would remove what you've added (I'm not sure why you added that or whether you fully understand what that does) and replace it with the content in the other answer:
'' => 'activity',
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