When you saw the question in the 2018 HSA interview that asked you if you "overfunded" your 2017 HSA, did you answer "no"? What this question is really asking is "Did you carry over any excess from 2017?" It will ask everyone this question, so it's not just you ;-).
As for the December 1, 2017 question, it also asks everyone about that, since some people could be like you and have HDHP coverage in 2017 and not in 2018. Yes, if you were covered by your HDHP every month in 2017, then you didn't need to use the last-month rule (which allows you the full annual contribution limit even if you weren't under HDHP coverage all year, so long as you were under HDHP coverage on December 1, 2017). So it sounds like you are fixed up.
