You know know when a planet is destroyed only if you or one of your corpies does a corporate planet scan. That and if you didn't notice it missing, you really shouldn't be playing the game...

As for the second scenario, if you are only updating corp planets, that wouldn't be a concern. If you are updating both, then it would update all info for the corp planet, but see that the others are not owned by you, and therefore just capture the names and the sector numbers.
If you have TWXSync capture all planet date for corped and personal planets, and either ignore enemy planets (easiest option for the moment), or only capture superficial data (a bit harder) you'll be able to add Syncing.
From the standpoint of an average player (I am not, I'm a lot worse), I would be interested in making sure I had available all information on my planets, and only superficial information on my enemy's. If I know where he is, then it really doesn't matter what planets he has in his sector, as long as I can keep him there.
