The only action acceptable would be to void the result and make them play it on a neutral ground behind closed doors.
This is the perfect occasion for them to send out a very strong message ahead of The World Cup ( UEFA and FIFA need to jointly agree to this) that any bad behaviour by fans, towards either, rival fans or players will be met with a zero tolerance policy, expulsion or bans.
Do you see Rugby fans behaving in this way? No, they even sit together at games, there’s no excuse for this whatsoever, therefore no excuses should be entertained.
I’d be even happy for City to take a stand and refuse to play the return leg as a protest, or put out the EDS, or a massively weakened team so that the title can be won on Saturday.
The team should not sweep this under the carpet, they should respond in the strongest possible terms and make a principled stand, UEFA will do fuck all, secretly they will be over the moon at City being all but out of the competition.