Please, don't do anything. Let TurboTax most of the things you have mentioned for you.
First , TurboTax will discover that your HDHP coverage ended in 2024. As you noted, if you had HDHP coverage on December 1, 2023, not only were you allowed to use the full annual HSA contribution limit for 2023, but you were also required to maintain HDHP coverage for the next year (2024). But, things happen, and your HDHP coverage ended in the middle of 2024.
To address your line items:
1. Yep, a common enough occurrence.
2. You will approach the situation of "failure to maintain HDHP coverage" (the technical phrase), but letting TurboTax walk you through it. Don't do anything first.
3. When TurboTax figures out that you failed to maintain HDHP coverage (because it (1) sees that you stopped HDHP coverage in August, 2024), and (2) when you answered "Self-only" to the question of "What type of HDHP coverage did you have on December 1, 2023?"), it will ask you a series of questions to calculate what you should have been able to contribute in 2023 if you would not have had the benefit of the last-month rule. But you don't withdraw the would-have-been excess - instead, the would-have-been excess is taxed at 10% in Part III of the 8889. No withdrawal is necessary (or even possible under the IRS protocols).
4. Yes, you will be able to contribute a prorated amount to your HSA for 2024. Don't pay attention to the "Calculate Max" button in the middle of the HSA interview - it is often mistaken (usually for taxpayers with Family coverage). But instead of proactively withdrawing the excess from your HSA, let TurboTax calculate the excess for you. Then, withdraw the amount that TurboTax tells you to.
I say this not for you so much, since you seem to have a better handle on HSAs than many, but for the many taxpayers who are unable to correctly calculate their excess HSA contributions.
When you actually withdraw the amount that TurboTax tells you is in excess (and assuming that you have the cash in the HSA to do it), be sure to contact your HSA administrator and ask to "withdraw an excess contribution". It is important that you use this phrase, so that the HSA custodian will record the action correctly. Note that some (many?) HSA custodians will have forms on their website to be able to do this request online.
5. When you ask to withdraw the excess contributions for 2024 (which you must do by April 15, 2025), the HSA custodian should (sadly, not all do) calculate the amount of earnings attributable to the excess contributions while they were in the HSA. Then, when they send you a check for the excess, they should add the earnings to it. Please do not do anything on your own with the earnings.
At some point, you will receive a 1099-SA in the mail (probably with a copy to your account on their website) for the excess and the earnings. You will add this to your 2025 return that you will do in early 2026. Note that the excess for 2024 - if contributed through your employer - was automatically added back to your income for tax year 2024, so you do not have to do anything about that.
6. The 6% excise tax (often called a "penalty") is levied only on excess contributions that are carried over to the next year (because they were not withdrawn). If you withdraw the entire 2024 excess, then you will not carry over the excess and so not be "penalized". If you have the cash in the HSA to withdraw the excess from 2024, do so, it is the easiest way of dealing with this.
OK, so the lesson is - let TurboTax do all the calculation for you, and let it tell you what to do next. This will be simpler and more accurate for you.
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