I think issue #1 is a big one, how to download a paid app without paying for it? Not everything is freemium.

Shattered Pixel Dungeon was a good example of me paying on one platform, and then applying the upgrade to the free app after downloading on another.

That's on the dev of the app. They should absolutely have the ability to see that you download the app, and allow full access again.

This isn't any different from a user switching to a new device and having to download apps again. Not everyone restores from backup and prefers clean installs. Downloading an app again should not be the point of friction