8 member online AVERAGE is the base at any given time (Let's say.).. And you want the shard to -1HP every minute. SO:
A shard will lose about 11,520HP per day at that average. If you want a 30D shard then 345,600HP.
This means smaller guilds if they manage to hold the shard, the shard will last longer. A guild with 4 members online average would be 60D. A guild with 2 members online is 120D. Obviously they probably won't hold a shard very long with only 2 members though.
For the guild war system, the rooms themselves won't be damaged, but if someone breaks down a door you'd have to repair the door, yeah. Same with dead NPCs. There will be an easy way to 'repair all' though.
Clan gold can get sacked (# * number of enemy that make it through, maybe a percentage - but to a maximum number).
Why use guild gold vaults then? People won't store gold in guilds anymore and will simply make a account that handles the gold and give access to all leaders. If there is a lossless way people will do it.