Hi Tutepani, I think the HBL person is mistaken. I had the same experience when I went to some banks and the people didnt seem to know or different people gave different information.
Finally I emailed the banks and it was forwarded to the right person.

You are able to transfer any part of the principal in us $$ back to your connected US account. Of course, you cannot send more $$ than you transferred to nepal from your us bank anyway

NRN Foreign currency account - both the interest and principal can be repatriated to the linked bank account in the US. You need to link your own US bank account with the NRN account in nepal
-----local withdrawals are in nepali currency