How can I set the a different font style (e.g. Simhei) for subtitle at Cover?
\mt2 \zvar|subtitle\*
and for "coverspine", the subtitle need to be vertical, and not cursive.
\mt2 \zvar|maintitle\* ~~-~~ \zvar|subtitle\*
On the Cover tab, click on the handy blue link(s) at the bottom of the screen to take you to the appropriate Styles for that part of the cover (in this case, coverfront).
Then, locate the exact marker(s) to be changed, and modify it/them as needed: