"A person divorces in 2020."

The 2020 filing is all that matters.

"Based on their 2019 MFJ return they were not entitled to EIP2."

That was only a projection.

"In 2020, filing single, she is. Do I claim the Recovery Rebate ($600)?"

Take what they got and divided it by 2. File for what she is entitled to, and enter her share of what they got, to see if there is a difference.

"A further pressing the limit...married they received $1200 for EIP1 which was split evenly."

Then she gets the value of $600 as already received.

"In effect she received only $600 when she would have been entitled to $1200 based on 2020 income."

Which is all that matters, along with her income limit.

"How aggressive would it be to claim $600 for Rebate Recovery for EIP1?"

It's not aggressive; the 2020 tax return is the reconciliation for both EIP 1 and EIP 2. Just enter what applies.

She is either entitled or not, for each.

She either got money (or the allocated share, once split) or did not.

There either is a difference, for each amount, or there is not.

That's all you are doing. There is no aggressive stance to take. Work through the numbers.