You had the value of the account at the end of 2020 and you know the amount that was excess. The IRS has a formula, and lots of articles also list it for you:

Net income = Excess contribution x (Adjusted closing balance-Adjusted opening balance)/Adjusted opening balance

You would have had this info by the end of Jan 2021.

"but now have it thanks to the 2021 1099-R coded J,P."

"Do we enter the income earned ( amount on 2a of 2021 1099-R) by entering a 1099-R as if we received one in 2020 with income amount so it will flow to the correct lines and forms?"

Code P means Previous Year. Did the entire excess contribution get removed? Reassigned to a later year? Not distributed in time to be treated as if it never happened (removed before the 2021 return was filed, but overlooked and not reported)? Is it still in the account? Did the earnings get distributed, with the excess?

Here is the ProSeries help article. Expand the part for how to report your codes:

https://proconnect.intuit.com/support/en-us/help-article/federal-taxes/entering-form-1099-r-proserie...

I like web articles:

https://www.investopedia.com/what-to-do-if-you-contribute-too-much-to-your-roth-ira-4770686

"the income earned on an excess Roth contribution in 2020 is taxable and subject to the 10% penalty in my case"

Penalty and Income tax on the earnings removed, and Excise tax on the excess contribution; it is taxed at 6% until removed, for every year it stays in the account.