"on what to do if married a couple was phased out based on 2019 income"

That is moot. That was used to Project payouts, and then issue the projection as an actual payout.

"and one individual did qualify in 2020." <== this is all that matters

The EIP 1, 2 and 3 were paid out based on Projections. For projection purposes, the earlier tax filings were used. That's why 2019 filings are Moot. You would split the actual amounts received to allocate it to each person. Each person has their own actual 2020 filing, which is the reconciliation of the amount entitled to (2020 actuals) against 2020 "prepayment" allocated to your divorced person.

Stop worrying about 2019 now. It has absolutely no bearing on anything now.