"and switched jobs/insurance away from an HSA to PPO in December 2023."
If you were covered by the HSA-eligible HDHP on December 1, you were HSA-eligible for December and eligible for the full annual HSA contribution. If your HDHP coverage ended before December 1 or your PPO coverage started on December 1, that would make you ineligible to contribute for December.
The fact that only $300 of the excess contribution was made to Account B makes it difficult to justify obtaining all of the return of excess contribution from only one of the accounts, so obtaining returns of excess contributions from both makes sense. To address your specific questions:
- Yes.
- Yes.
- Yes.
- Yes, this ~$20 is reportable on the tax return for the year of the distribution, 2024.
- The $300 is simply not deductible on your 2023 tax return. The code-2 2024 Form 1099-SA from Account B will show only ~$20 taxable.
