As far as I know (which could be completely wrong, and I also might be answering your question far too late), "Spiser du jordbær" translates to "are you eating strawberries" because "are you eating strawberry" doesn't make grammatical sense. You would determine the plurality of the word based on context.
If the word does not have an 'en' either before or on the end of itself, then it is in the plural. Examples: 'en jordbær' is 'a strawberry', 'jordbæren' is 'the strawberry', but 'jordbær' is 'strawberries' because it lacks the 'en. Please be a little more hesitant before hating on something someone took time out of their life to make, without obligation.
Not all languages have the same alphabet. English and Norwegian both use the Latin characters to some degree, but languages change over time. English used to have other characters, like 'thorn', which was 'th'. Considering other languages, Norwegian is quite easy to learn the alphabet of. As to your second question, I have not encountered it, but I think it might just because some Norwegians decided it was easier to say, but didn't spell it differently. Hope this helps!