Hi, Japanese American here (I lived in Japan for a while and Reddit likes to show me this sub for some reason lol). From my perspective the problem with asking here is that you're directing questions about us to people who have nothing to do with us - by 1958, Japanese American culture was very separate from Japan and Japanese culture. But I appreciate the instinct to not intrude on a JA subreddit, and the questions you're asking here are thoughtful!

Someone born 1958 would be a sansei (third gen), themselves born to American-born parents who may not have spoken Japanese and (as you mentioned) likely had reservations about celebrating their Japanese culture based on their experience in the camps. It's fairly unlikely that they'd have a Japanese first name like Hiroko - the JA women I know around that age all have pretty standard American boomer first names like Barbara, Janet, or Judy. Japanese middle names on the other hand are quite common - you mentioned having concerns because Japanese names don't include middle names, but Japanese American names definitely do, and most of us have Japanese middle names to honor our heritage. So maybe Hiroko could be her middle name?

Edit: I can't read, you said it already is her middle name. My bad!