1) Every change that you do trough the ispconfig interface on the master server gets replicated to the slave. Technically, the slave is querying the master once a minut if there are pending changes to be replicated.
2) The ispconfig sources are always installed completely, but the interface is not activated.
3) Changes cause only minimal loads as only the changed record gets updated and the system is completely fault tolerant. Or how often a day do you change the settings of your mail account (I change mine typically every few moths if not just once a year) if you compare it with how many times the mail table has to be accessed when a mail is received. Thats why it uses exactly this architecture and not a central DB! You can power down the master db or power down the slave db, it does not matter. As soon as both db's are online again, the slave will get synched. You can even power down the slave for weeks and nothing get lost.