What happens to Joplin data if its processes were terminated while editing or syncing?
It should be fine, if sync is interrupted it resumes the next time. And save cannot really be interrupted since it's atomic - either it was written to disk or it wasn't.
The only thing is that if you type something and it crashes within the next second or two, you might lose what was typed.
I want to synchronize notes and profile together using a standalone cloud sync app, so in that case I shouldn't install Joplin on two PCs with same profile folder in sync?
You can't sync the profile, you'll corrupt the data. Instead you should sync using Joplin's own sync feature
Does the sync feature synchronize everything? Including custom CSS, plugins, notebooks, tags, and settings?
Only notebooks, notes, and tags are synchronised.