The point is that Duo only has a certain number of words programmed for each translation. I guess granny just doesn't happen to be one. You're right about it being dated-although my grandchildren still call me "granny'. Anyway stick to the more usual vocab to get through smoothly. See here for some tips:

https://www.duolingo.com/comment/4821654

It should probably be accepted; it's certainly correct. But past a certain point, it gets to be too much work to add in every possible synonym.

Granny grandma nan nanny and gran should be excepted

I'm granny.

Yes it should be grandmother, if you get this again report it and it will be corrected, however, the corrections can take a long time.

Ogni is not plural. It means "each", and it comes from the Latin omnis. Just as in English you wouldn't use it with plural nouns such as "each apples" or "each men" or "each days", in Italian you don't say "ogni mele" or "ogni uomini" or "ogni giorni". Like "each", ogni is used with a singular noun, not a plural.