"Where can we buy a phone card?"
Yes, though the measure word in this case may be 张 zhang for a card. I am wondering why many of these exercises omit measure words.
No for this sentence. Normally the adverbials like “在哪儿” do not come at the end of a sentence. For this sentence, you can insert it before 我们, 可以 or 买.
I don't know if the grammar is correct but you will still be completely understood. My grammar is terrible and I am still understood most of the time. Trying to improve it here!
It's the difference between "where do we buy" and "where can we buy." Most of the time, the phrases are interchangeable, but not always.
Where are the measure words in this and many other exercises?
Sometimes Duolingo requires the time words to come before the main verb, and other times it requires the time words to come at the beginning of the sentences. Why? Which is correct?
I'm not sure where the time words you mentioned are in this sentence, but generally I find the Chinese sentences correct and it's the English translations, or the pool of accepted ones, where I find an issue.