The performance is only an estimation. Even the advanced computers can’t calculate that clearly. Therefore the performance might be better or worse than the calculated. This was the case with the 777-300ER, obviously this was the same with the A345.

It, of course depends on the operator, but with normal flag carriers can operate profitable when half of the plane is full. For example, SNBA is loaded for 55% and made a small profit. With ARJ’s not being the most efficient aircraft, this is a whole achievement.
However, SIA offers more service and has higher overall costs, so I’d say 60-70% for this airline.

Aircraft usually have excess cargo space. Therefore, other cargo not belonging to passengers is also taken. This is more with Long Range aircraft. If the airline chooses not to take other cargo, the aircraft can take off with a few tonnes less – reducing fuel consumption.