There was a change made in early MC17 beta's, which the devs describe as:
WM_CLOSE should do the same thing as Alt+F4. So if you're in Display View or Theater View, this should fall back to Standard View.
To close Media Center completely, you can use MCC_EXIT (20007):
MCC_EXIT, // [int nMode (0: normal, 1: force close (close media server), 2: force close (allow media server))]
I suspect that Mini-view is similar to Display or Theater View, in that the close actually returns to Standard View, whereby you could hit Alt+F4 again.
If so, you can program a key to close entirely. You can test this in a command line with the command:
mc17.exe /mcc 20007
Ask if you need help setting this up and want to proceed.