Mum and dad both ends on -a (mamma og pappa) so from the native speakers point of view they have to be masculine, if not you could not differentiate between the definite and indefinite form in singular. In Norwegian we add stuff to the end of the word, rather than saying a car - the car we say en bil - bilen. And we never ever end a word with two aa's.
True, DL will ignore minor spelling errors, which I think is a mistake. I once went through an entire unit spelling "fotball" as "futball", and DL never complained once. I only noticed that I was spelling it wrong when I went to a discussion and saw it spelled as "fotball". It's probably an overall DL thing that can't be adjusted at the course (language) level. But it would be nice to at least get a warning that you are misspelling things.