You don't say what actual version of software you are using?  17.6 could mean anything from 17.6.1 to 17.6.8!Either way, 17.6 is almost end of life - it has already passed the End of SW Maintenance and End of Vulnerability/Security Support dates!  So...

Just to add my bit - sudden disconnection of all clients on an AP is most likely going to be :- radio crash/restart- AP crash/reload- AP CAPWAP restart causing AP to re-join the WLCshow ap uptime will quickly show you whether the AP has reloaded or r...

Yes, it is possible, but like @Saikat Nandy I would recommend one at each branch which is how EWC is intended to be used.