I think I've discovered a work-around for the Graphic Window resizing issue. If I unplug the HDMI cable form the Mac Mini to the 22" HP monitor and then plug in back in, then the Graphic Window can be resized to take advantage of the iMac's 27" display.
I also discovered that if I switch from SharpGray to Golden the Graphic Window will automatically stretch across both screens and beyond but just as a shadow on the 22" HP display. If I then click the green resize button the Graphic Window disappears but I can get it to come back by turning screen sharing off and on and it's then back resized for the iMac's 27" display. If I do this again Metro will get confused and the Graphic Window becomes empty and twitches. A forced quit and restart of Metro does return it to it's original setup with VE Pro 5.