I couldn't find systemd services named powerdevil kded or kcm so I turned power management off in settings, and killed the kded and kcm processes. Here's a chart with Each of the commands you wanted me to try with the ways I tried to wake the screen. The desktop always returned to a normal, usable state after the panel crash, but with a crash dialog open, though sometimes it immediately disappeared.

command...............................power button......................................................tty switch..........................normal key

xset dpms force standby.......panel crash.......................................................panel crash.......................sometimes works, usually blank screen
xset dpms force suspend......panel crash.......................................................panel crash.......................blank screen
xset dpms force off ...............blank- panel crash if I then hit a key............normal desktop*.............blank screen

*brief "shatered glass" effect

run "kcmshell5 kded", there you can disable powerdevil. This has nothing to do with systemd...
DO NOT KILL KDED! EVER!

The suggested xdpms behavior implies that the output unregisters as soon as it enters some powersaving state, causing an xrandr event, causing the segfault in QScreen - this won't fix before Qt 5.6 then, sorry

OK. looks like I'll have to live with it until march, unless there's an easy way to run a beta version of it. Thanks for your help- at least I now know this is being worked on.