OK, let me restate what I hear you saying. You were not under HDHP coverage for every month in 2018, but you were under HDHP coverage for the months of September to December, correct?
Yes, the last month rule states that if you were covered by an HDHP on December 1, 2018, then you are entitled to use the full annual HSA contribution limit. When you talked about the excess, I assumed that you meant that you had received the message from TurboTax, but apparently you did not.
You are correct that using the last month rule requires that you stay under HDHP coverage for all of 2019. However, 2019 is not yet finished. If indeed you leave HDHP coverage (and it sounds like you already have), then on your 2019(!) tax return in early 2020, TurboTax will ask you some additional questions to determine what your HSA annual limit should have been in 2018, and will assess you according. You do not make any adjustments in 2018, even if already you know that you will fail the "testing" period in 2019.
