Rex Grossman

That's just true random encounters. As far as I know, the singular events like gifts or the Tepig at Pyrorus have stayed consistent across episodes. It also means without outside assistance, you're stuck with just the guy you pick at the Grand Hall for quite some time.

Was it on a Pokemon that you traded, or gave away for a sidequest? Did you scour your boxes? Check the daycare?

The Dratini in Reborn are true random encounters, so they don't have random egg moves like the interactables. I think you're SOL on that front.

The Wasteland Wall, accessed via the Tanzan Mountain tunnel that needs the Crystal Key.

E18 Error report: I found a heart scale when mining at the bottom of Mt. Tanzan, but it called the incorrect part of Items.png. It appeared normally in E17.