Sorry for not beeing precise .. with master/slave I mean active/standby = 1000APs on EWC#1 and no APs on EWC#2 - if #1 fails #2 will take over.
Some customer like that setup even that would result of using only 50% of the max. possible AP count on the controller pair ... i.e. customer want to avoid inter controller mobility
3. There are some general settings that you'd need to setup on both controller individual but the main WLAN/SSID configuration could be done on either one of the controller pair and is synchronized to the other controller (if sync is enabled).
If you have more HA pairs and also have Netsight you'd use the adaptive UI = access all the controller GUIs in one window instead of have 6 windows open - you'd see a screenshot of it in this topic...
https://community.extremenetworks.com/extreme/topics/using_netsight_5_to_manage_wireless_controllers