Surely sync checks for differences?
Hopefully it does check for differances when importing the ICS file as you say, but I’ve personally never tried it before to verify if it does or not. I’m not game to try it on my Google Calendar.
Hopefully another user or if your lucky eM Client support may reply on this thread if it does.
Yeah, give my experience to date I don’t think I’d risk it either! 
It’s a shame as the email side of the program is excellent…
For some time now, Calendar and contacts are not syncing with the proper gmail account. Coincidentally (?) my Android isn’t syncing completely either.
What worked for me was to
(a) open Calendar view
(b) under my email address on the left, right-click Calendar
(c) select Properties
(d) in the “Calendar - Folder Properties” window click on the Repair tab
(e) click the Repair button