To answer my own questions.
- Re: Icon Sizes: Never mind. They are good. In general, for these I would reference the Asset Catalog "AppIcon" sizes in Xcode instead of any online references as these may go out of date (as the above one did for iPad Pro..).
- Re: Icon Names: This does not matter so much. Names can be arbitrary, and I think @RoadkilleR78's names are fine (using the point size) as it makes it easy to drag them into the correct icon "wells" in Xcode
- Re: Exporting. In Export Persona > Slices palette, select slices (checkmarks) and at the bottom of the palette select "Export Slices" and save to the Assets.xcassets/AppIcon.appiconset folder. (Ignore "Batch Builder" in the Slices palette.) Then return to Xcode Asset Catalog and make sure these icons are dragged into the correct icon "well".
One other tip for exporting. Make sure the names of the slices include the "@2x", "@3x" etc, and slice scaling remains at "1x".