I'm not 100% how to interpret the first sentence in Russian.
Is it something that one could say in answer to Кем он работал в прошлом? ("What kind of job did he do in the past?"). If that's what the sentence кем только он не был в прошлом actually means, then you could translate it with "What kind of jobs HASN'T he done!" or "What HASN'T he tried!" or "Who HASN'T he been!", etc.
The second sentence is easier for me:
"No matter who he used to be, it's unimportant to me" (or: "No matter what he used to do, it makes no difference to me" or "I don't care what his old job was")
Of course, the best phrasing would depend on the original question that you're responding to. (Are you asking about his professional career, or his criminal record, or his sexual history, or whether "he" was actually born as a biological female, etc.??)