“In order to be saved, one must simply accept Jesus as the Son of God. That’s it.”
Satan acknowledges Jesus as the Son of God.
To be saved, one must confess their sins and repent of those sins and submit to the authority of Jesus as supreme in their life.
That is dying to one’s own sinful desires and being born again a new person under the authority of Jesus and free to live a life without the condemnation of sin.
What if they sin again?
God never offered salvation to fallen angels so it doesn’t matter that Satan recognizes Jesus or whatever.
A born again believer who sins is still a born again believer who is still going to heaven.
Salvation is not on again, off again, depending on how much I sin or not sin.
Because the long and short of it is that EVERYTHING about us in the flesh is corrupted by sin and we can not not sin.
Only by the grace of God who forgives us ALL our sins, are we saved. He cancels the written record of debt that stands against us, so there is no sin left to pay for and suffering is not the way that happens anyway so no stay in purgatory is necessary or will work.