1) Is outlook web access for devices a feature available for both on-premises exchange servers, or just office365 online servers? do you know which mobile OS are supported by the OWAforDevices?

2) Out of interest.... aside from active sync, outlook web access (website), OWA for mobiles - are there any other "remote access" type facilities for accessing an Exchange mailbox from personal/mobile devices?

3) if the output of get-casmailbox indicates OWAEnabled = FALSE, and OWAforDevicesEnabled = TRUE, does that mean users can connect via OWAforDevices even though OWAEnabled is FALSE and thus blocked for those users. 

1) Yes, both EO and on-prem Exchange 2013. iOS and Android are supported

2) The new Outlook app: http://blogs.office.com/2015/01/29/deeper-look-outlook-ios-android/

3) OWA for devices uses ActiveSync, blocking OWA access will have no effect. The only way to block OWA for devices is to set OWAforDevicesEnabled=false.