I would have accepted it if it was something people would actually say, even if it was written like two words or more. Example: sister/brother in law is something that is close enough even if it isn't written in one word like Norwegian svigerinne/svoger.
But as far as I am aware English only uses grandsomething (or variations of it) + further description as needed.
In everyday speech you generally do not try to be this specific, but if you wanted to (e.g. recounting family history to a doctor or talking about the relationship between your parents and granparents) you could use them to be more specific in a clear way.
I agree that almost every use of farfar should be simply translated as grandmother.
English speaking people do not use these all that often. They say "grandmother" or "grandfather". They specify which side of family these come from only when they really need it for some reason.