I share some of your concern with loadsheets, but I can't see a quick way round it.
Just to add to the confusion/ facts.
Air 2000 use the figures you mentioned when the flight is a scheduled flight. For charter flights the passengers weights are 83 for a male and 69 for a female.
In the UK I think all the loadsheets show the actual weight of bags. Down route I presume they don't have any equipment to weigh bags so provide purely number of pieces.
Just to make it even more interesting. If its a scheduled flight then scheduled weights are used, but often there may only be a handful of scheduled passengers on board, however all passengers are counted as scheduled for the loadsheet. So we know its pure fantasy figures.
Unless every passenger and bag is weighed we'll never know the true figures. But we don't usually have a huge discrepancy between the loadsheet actual weights and what it would have been if you'd multiplied the number of pieces by 13. From memory luggage is usually around 3000kgs. Of course it does vary depending on the flight and things like ski flights have weights calculated slightly differently.
As for upping the taxi fuel to make the max weights in limits, then why not? As long as you leave with a legal loadsheet then you are OK. You can never know exactly how much weight you have on board.
Of course, if you are severely limited, on a short runway with terrain problems, or something else to consider then it becomes particularly important to ensure you are not overweight.
As for Air 2000 trim on the 757, I don't know why the policy is there, all I can think is it may be something to do with the policy Air 2000 has of trying to make the Boeing and Airbus operations as similar as possible.