I think the invisible planet solution is a mistake. If a player doesn't join shortly after a game forms, they may die, because a nearby player could take the planet. The reason a game has a start is to get players to join at a specific date or near that date to start playing.

I don't think any player would have the power to take another player's system for a week (can't you give home worlds unclaimed a higher pop for defense?). If you don't join in a week, then well, your loss.