1. Could the public be asked to re vote on Brexit?
Yes, and I hope that we do get the opportunity. However, the real question is about what exactly we'd be asked to choose between.
2. If the new vote favoured 'no brexit' would the EU have to accept we then wanted to stay!? Or could they demand we leave because we've pissed them off?
No, we probably have pissed them off but we can unilaterally revoke Article 50 without the agreement of the EU27. On balance, despite the huge amount of time and money that has been wasted on Brexit, I think they'd welcome the decision.
3. Could parliament decide to vote to cancel Brexit by themselves!? Or would it have to go back to the public?
Yes, parliament could just decide to cancel the whole shit show without going back to the public. However, politically, that would be very difficult. I think there would need to be a clear mandate for revoking Article 50 - either a vote to remain in a second referendum or a general election which was won on the back of a remain manifesto.
4. Do you think a new vote to remain would lead to actual riots?
Probably not. There might be a bit of civil unrest/violence from far right groups seeking to exploit the opportunity, but I don't think it will be widespread. I suspect there would be a much greater risk of riots if we crashed out with no deal.