An injured player would probably be on the roster and attending practices.
In 3A and 4A, playoff seed is based on region record. I like the idea of having players be rostered for a minimum number of region games. If a player is hurt, they can still be listed on the roster since their is no maximum number of players that can be listed.

If a player moves in or any other circumstance, they should have to file a petition to the high school league to be approved. From the sounds of it, it sounds like it's a school decision.
Our school got fined and wins vacated because we had a swim coach that teaches at Stratford, had her middle school daughter on the team but they live in a different attendance zone, and the school didn't file a "hard ship" for her to be eligible. I know that isn't the same thing but the high school league does hear appeals like this scenario.