Rosa did suggest they were Peace Corps-types when talking to me about it. And yes, the dates don't work for the actual Peace Corps.
Really interesting, I didn't know about that... though as you noted, they weren't founded until 1961 so the dates don't work. But I am not surprised since what Rosa says vis-à-vis while answering questions during conventions is obviously more spontaneous and less rigorous. I mean, it's not like he will stop answering mid-question to take his cell phone and do an online research to see when the Peace Corps were established. There is also the matter of context: I don't know how well you remember his exact words, but it's impossible for those who were not there that day to say if he actually meant the Peace Corps themselves or just some group like them.
Anyway, the common denominator in these various statements is the idea of HDL's parents being lost in a valley, possibly Tralla La.
I thought of another alternative for a "HDL search for their parents"-story. How about you set the story in the near-future, when HDL are adults? Then, they'll be old enough to already live on their own, so choosing between living with their parents and their uncle becomes a non-issue. Adding Della and her husband to the cast wouldn't even upset the status quo, since this is the future, and that's already non-status quo-territory.