I am a fluent speaker (not a learner) but I can't explain why が and は are used in these lines when asked by a friend.
そんな私を拾ってくれたのがその時言い合いとなったお客様シェフなんです (from this video)
それを身を挺して助けてくれたのが誠司さん (video)
I feel like は seems more common in similar contexts: そこで現れたのはこの子です
どうすれば記憶は戻るんだ (vidoe)
With this one I think it's more common to say どうすれば記憶が戻るんだ. I can't explain why は is used here. Google shows どうすれば記憶は 15, どうすれば記憶が 3,740 proving that my intuition is right.