With regard to #3, if there is any investment gain required to accompany the return of contribution, the gain will be taxable.
With regard to how HSAs and HRAs interact, I suggest reviewing IRS Revenue Ruling 2004-45:
https://www.irs.gov/pub/irs-drop/rr-04-45.pdf
I believe that you would have had to suspend the HRA before the beginning of the HRA coverage period to be eligible to contribute to an HSA during the coverage period. You are only an eligible individual for months that the HRA was not available to cover qualified medical expenses.
If you are an eligible individual on December 1, you can make a full-year's contribution to the HSA under the last-month rule. However, the last-month rule requires that you remain an eligible individual throughout the following year, otherwise there amount contributed that you would not have been eligible to contribute were not for the last-month rule becomes subject to income tax and a 10% additional tax (but is not an excess contribution).
