Verona: However, upon reading your comments on #1 I became uncertain about reacting to one's innocent "What did you do last night?" "I was studying [English]", I guess It's the discrepancy in the question/answer tenses that makes me feel a bit uncomfortable.
5: In coursebook exercises, the assumption often appears to be that questions are always answered with the same tense. That is largely true of short answers:
Were you working? - Yes I was/No, I wasn't.
Did you work? - Yes I did/ No, I didn't.
However, it is not necessarily true of longer answers. We respond with the tense that reflects what we wish to say:
A: Did you work on your book yesterday?
B: I was working on it till past midnight!
Verona: The difference between wrote/was writing a novel (#3) is also clear to me. I do understand what "I was working on my novel" implies, but I did't get which alternative "worked/was working' you considered preferable. I think you meant both of them were fine, and it was just my comments in brackets that weren't convincing enough.
5: That is what I meant.