You said it yourself. You did not qualify for citizenship if you naturalized earlier than 3 / 5 years anniversary of being LPR.
It's all about how many days you had at the time of oath, not after.
One would hope so. But it depends on the judges. If they interpret law strictly, it can be taken away. As an example, during pandemic, some people received stimulus check by mistake. It was a government's fault. But yet IRS chased people to return the money. Same here. Benefit was given in error. It can be taken away.
Theoretically it can be taken away. Naturalization occured before one was eligible. This is how the law works. It looks at the facts as of the day of naturalization.
There were cases of people getting US passports in error and then, denied passport renewal decades later.
There also may be other downsides. If this person tries to sponsor somebody, USCIS may claim they're not eligible to sponsor somebody.