1. and 2. It's likely the case that the amount in box 1 of the code 8 Form 1099-R is the sum of the excess contribution and the attributable gains. If so, I would expect the taxable amount in box 2a would represent just the taxable gains earned while the excess was in the Roth 401(k). But yes, it would be good to confirm that with the plan administrator.
3. The amount that had to be treated as a return of contribution from the Roth 401(k) due to failing the non-discrimination testing and was not eligible for rollover is now an excess contribution to the Roth IRA, at least to the extent that it exceeds what you could treat as a regular Roth IRA contribution for 2020. Assuming that you treat the entire amount on the code 8 Form 1099-R as an excess in the Roth IRA, you must obtain from the Roth IRA a return of contribution (not a regular distribution) of the excess. It often takes a bit of effort to explain to the Roth IRA custodian how this came to be an excess contribution instead of an eligible rollover contribution so that they will actually perform a return of contribution. Once convinced, they distribute the amount to be returned, adjusted for any investment gain or loss, reported on a code JP 2021 Form 1099-R. Any gains included with the returned contribution will be subject to tax and to early-distribution penalty on your 2020 tax return.
Odd that the code 8 Form 1099-R does not also have code B if the distribution of the excess in the 401(k) plan was in the designated Roth account. Code 8 by itself would suggest that the excess was instead distributed from the traditional account in the 401(k), which doesn't seem to make sense unless you contributed both elective deferrals and Roth contributions to the 401(k) from your pay. Does the code 8 Form 1099-R show the same amount in box 2a as is in box 1?
