都 refers to each single day, where the action is performed.

很 heightens the extreme of the adjective. In the sentence 很努力 refers to "very hard work", which is being worded differently in the resulting sentence.

According to https://www.duolingo.com/skill/zs/Health-1/tips "Wǒ měitiān dōu duànliàn." translates to "I work out every single day." Since there is no "single" in the English sentence, the correct Chinese translation should be 她每天很努力工作 and 她每天都很努力工作 is wrong.

I agree. They did state in the tips and notes at one of the first few units that they'd be requiring it as a general rule, and I understand their desire for uniformity (here), but I think this sometimes does more harm than good, especially where the sentence flows naturally without it.

I was wondering the same thing..

Yes, but you've a typo: it should be 做得很努力.