I always assumed the angry son was placated by his father’s remarks, and went in to celebrate.
I'm influenced in that, I think, by the utter thickness of the Apostles. Despite three years with Jesus and then the coming of the Holy Spirit, they still could not imagine the Gentiles' welcome into the kingdom (forehead smack: "So, even the Gentiles!").
As if God, Who IS love, would confine His salvation to a tiny portion of humanity while consigning the rest to outer darkness. What hubris!
It reveals how profoundly the prideful hearts of the Jews were dead-set on not sharing their favored position with God. And it suggests why Jesus would address this problem in His parable: it seems more pervasive and troublesome than early believers' gripes about those scrambling in late--which, as you pointed out, Jesus addressed elsewhere.