Regarding question #1, you can only do this if the orthodontia expenses were incurred after the date your HSA was established, generally the date that the first contribution was made to the HSA.
With regard to #2, the proper way to do it is a HelenC1 said, ask your HSA custodian to accept a return of mistaken distribution of the $118.99. You can then make a new distribution to cover new expenses. While the result would be the same as just applying the $118.99 directly to the new expense incurred in 2020, the IRS has never issued any guidance indicating that just applying it to the new expense is permissible. I probably wouldn't worry too much about it if the new expense had occurred in 2019 after the original $600 distribution in 2019, but I would be a little concerned about doing so with a new expense incurred in 2020.
