Translation:They are already full.
Yes. I agree with you. They may add "They all are already full" to make it consistent since 都 is there.
Yes, but I think he means that 很 is often used in a sentence even though the speaker isn't emphasizing "very". Looking at the "hover over hint" shows 很 can also = is or be used to connect adjectives. From many conversations with native speakers from both Taiwan and the mainland I can confirm that 很 does not always mean "very". That said, translations of this sentence either with or without "very" are both "correct" and currently accepted.
I've got several answers wrong for not translating 都. Now it's here and it's not translated. I demand consistency