Probably not, it's been trading for a year so surely people have bought and sold and bought something else and withdrew etc... I doubt it could be untangled with a rollback. Livecoin really made in unnecessarily complicated by allowing a known bad coin/fork to be traded for so long.
I would argue that if you were buying MONA after the fact, you knew what you were getting yourself into (pure speculation since price was so much lower). Since there apparently was/is a market for it I don't think any of the users that currently trade it have lost anything. But I do agree with you, however nothing can be done about that now so I'm trying.
Some things also get lost in translation I feel which makes it increasingly more difficult. I'll keep people in here posted though.
If it was bought after the fact then the exchange would have known that they were selling tokens that they did not have possession of. The buyer may not have had such knowledge. As soon as the exchange became aware of the issue they should have frozen the markets for that coin.
While an exchange is not responsible for the MONA network and 51% attacks are hard to detect in real time - the exchange should freeze the markets as soon as they become aware of a 51% attack or attacks.
Allowing trading to continue is effectively continuing to sell something that you know that you do not have.