"You keep offering up responses stating that a taxpayer who died in 2020 is eligible for the RRC."

Yes, because there is no Splitting of 2020. They either qualify, or they don't, based on the 2020 tax return. Not part of the return or only a few months of the return. It's an Annual Tax Return.

And you are reconciling what they are entitled to against what they got.

The change in Unemployment is going to affect some taxpayers' qualification now, as well, if they were pushing the income limits.

You can do your own research, and you should do it for the third advance payment.

Perhaps it would help to review what is really happening:

The funds were paid out as Advanced payment against a projection. The projection used 2018 or 2019 tax returns. But 2020 is the Actuals. You use the 2020 return to reconcile what a person is entitled to, against what they got.

If the person is not a dependent in 2020, then they would be entitled to the payment/credit as individual filers. That doesn't mean "not being claimed." It means "no longer qualifies as a dependent."

You might want to bookmark these links and read the IRS guidance.

Interactive wizards portal for determining dependency:

https://www.irs.gov/help/ita

And:

https://www.irs.gov/newsroom/economic-impact-payment-information-center-topic-a-eip-eligibility

https://www.irs.gov/coronavirus/second-eip-faqs#Eligibility

One for each EIP.