I would agree that the specific sins were the context: those crucifying Jesus, and those stoning Stephen.
That jesus' request was granted is plain, because He is God, and He Himself forgives sin. No reason to believe it wasn't the case here. Though I don't see in the context of it having to do with any other sins, or their general lost condition. Though at least one present DID, indeed, find faith and salvation in Christ, the thief next to Jesus.
In the case of Stephen's prayer, we can't know whether this was forgiven or not-this is between the Lord, Stephen and those stoning him. It is possible they weren't forgiven for this, unless they found salvation, but I suppose we'll know one day. It wouldn't have mattered if they continued on in sin, except that, if they were forgiven for this one, perhaps their eternal jucdgment will not be as bad as if they were held against them.
It is possible some at the cross were converted because:
Matt 24:54 Now when the centurion, and they that were with him, watching Jesus, saw the earthquake, and those things that were done, they feared greatly, saying, Truly this was the Son of God.
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