I type ChangJie/Quick on Windows, and so far I'm not aware that the "Z" Key is labeled "重", probably because I use US ANSI keyboards... And because of muscle memory, most ChangJie/Quick typists can type them on any keyboard.
The Z-key for standard ChangJie/Quick is for typing punctuation and miscellaneous characters.
The X-key, normally labeled as "難" (lit. difficult) is for filling in strokes that are not represented by any other keys.
FYI, touchscreen keyboard layout of ChangJie on Windows 10: