Do Mandarin people use 補鑊 as slang to represent fence-mending or damage control?
補鑊
fence-mending; damage control; get a problem fixed
This term is used in Cantonese, not Mandarin/Standard written Chinese

補救 (remedy) can be used in both Mandarin and Cantonese
Example:
事發後才去補救已經太遅了 (Mandarin)
事發後先至去補救已經太遅啦 (Cantonese)
事發後先至去補鑊已經太遅啦 (Cantonese)
~
現在補救還來得及 (Mandarin)
而家補救仲趕得切 (Cantonese)
而家補鑊仲趕得切 (Cantonese)
補鑊 and 補救 are mostly interchangeable in Cantonese