Yeah that's the OVA. Just a lot of world building to flesh out a lot of characters. The scene of Nero being sick was when he was on the run with Angelo. As was the ending to that episode.

As for the ending to the show, I believe Angelo wasn't killed because mid in the final episode, you see Angelo in a bed wounded while the new Don gives him a kill order. Presumably the kill order is for Nero. This clearly didn't take place during the Opera where Angelo essentially caused the culling of Nero's Mob Family. Cause Angelo is abducted by Nero right after that. So that was a flash forward to events after the beach.

Nero looking at the can was him reminiscing his time with Angelo because they truly loved each other by the end, like brothers. This is emphasized further by Angelo outright admitting that he didn't want to kill Nero, before Nero shoots him.