D.L., CasselberryA: I believe your mom will be able to keep the stimulus money paid to your dad.
Since the money is a credit against 2020 income taxes, your dad would qualify because 2020 will be his final tax return (jointly with your mom).
And while the IRS has not issued clear guidance as of this writing, it has said that parents who received stimulus money for children who turn 17 during 2020 can keep the funds, even though the payments for kids were only for children under 17.
Chris ToadvineQ: I have not filed my 2019 tax return yet, can I still make a Roth IRA contribution?
Due to COVID-19 and the federal CARES Act, your personal income tax return has been extended until July 15, 2020.