You are overthinking this, because you included a ton of details here that do not relate to the issue, of the RRC (if that is all you want to work on).

"But did claim the Student for 2020 & 2019 and not for 2021." <== this

The payout in 2021 was against a projection of 2021 tax filing, but it will have used whatever was the most recent tax return on file. For these parents, then, that is either 2019 or 2020 (payments started going out while 2020 tax returns were not yet due).

What we've seen a lot of is that 2021 actuals where the young adult becomes no longer a dependent (not claimed and cannot be claimed by the parent(s)) the IRS still is matching to those projections. They seem to expect that the families will fight it out over the fact that the parents got the money, but now the young adult qualifies under actual reconciliation on the 2021 tax form.

With your facts, the parents would have gotten the funds, but didn't qualify (limited due to income). Both the parents' 2021 tax return and the young adult's 2021 tax returns should be filed as applicable, and then the young adult qualifies or not on their own. You don't add a dependent to the parents' return, if this young adult does not qualify as a dependent. Or, the young adult individual filing for 2021 reflects that they still could be claimed, even though they are not being claimed, as a dependent by the parents.

It's the saying: Pick a lane and stay in it.

Don't try to fake out the 2021 returns. File them as applicable to 2021. If the IRS adjusts incorrectly, you then have something to stand behind.

But, if the advance payment got made for that young adult, the IRS seems to be doing a better job of avoiding a redundant/duplicate payout. All of these families need to fight it out amongst themselves.