I think the Spoils Of War mechanic sounds promising, particularly if the Spoils are relative to the amount of time the previous owner has held the planet.
That seems like the right idea for defenses, though you need to ensure there are always enough dropships in circulation for even a losing clan to make an attack if they wish. Sending dropships in defense sounds good as well, as long as that is logged in both the general and planet event logs.
I think there may be work to do on the planetwars balance algo. Simply speaking, it kept setting up absurd games unbalanced both in player ability and in player count... a match with Google, Rafal and Maackey on one team of nine against a team of seven with Waldo is hardly a match at all. After two matches with this sort of balance (URFL + Others against DOOP + Others I assume?) half the players left the server, including most of the major clanned players.
Part of the problem is the clan composition, I understand playing with friends but some clans are simply too much better one for one to make clan conflicts very fun. I kinda hoped that in this incarnation of PW we would see the highest (Top 10) ELO players establishing independent clans and then drafting from the lower ranks, but instead we have a few clans with multiple Top 10 which are very difficult to best. I don't have an easy solution for that, besides more players.
Anyway, it might be good to weight ELO and team size more and clans less, at least it should yield better battles. The metagame will be awkward, though.