I don't understand your first question. Are you assuming it should be something else? Not all normal native speakers of English learn grammar rules. Here 'we all' is the subject phrase. In your second, 'all' emphasises that everyone went. I think that's directly equivalent to 都 - 我们都去了...

Answer to 2 We Japanese learn the grammar "we all" as 同格. I think you can understand "we Japanese".

I actually got confused, but I would just comment hoping that you will be able to confirm its correctness. "All" can be an essential appositive so it's not set off by two commas. That's the best answer I can think of. I hope it helps.

this question is in a exercise book and i get it wrong. the answer is S+V+A but i dont know if A stands for adverbial or adjunct

also 'why is the sentence structure...'