The Advance credit is just a pre-payment of half of the actual credit on the 2021 return, so the regular rules apply to the actual credit. 

The IRS will be sending the pre-payments based on the 2020 tax return. So if the person claimed the child in 2020 but will not claim the child in 2021, that parent will likely receive the Advance credit unless they "opt-out", and if they do not "opt-out" they will need to repay the Advance credit when they file their 2021 tax return (unless their income qualifies for the "Safe Harbor" to reduce or eliminate the repayment).

Whichever parent claims the child on their 2021 return will get the $3000/$3600 regardless if the other parent received Advance credit or not.

As for the parents that want to revoke the 8332, you may want to inform them that if their divorce decree says the other parent can claim the child, they would be violating state law and could be in contempt of court, so they should consult their divorce attorney before doing such a thing.