when you "file > export > to pdf" go to the fonts tab and make sure that all you fonts are outlines or embedded.
if it's the case, then the font should really be in there.
if they're not it's a bad scribus bug that needs to be fixed asap!
but i'm pretty confident that if you did not miss a font there, scribus will be doing the right thing.
and, no, scribus does not have pdf/a...
(and pdf/a is not meant for the print workflow but (as the a suggests...) for long term archiving of documents)