I don't know how anyone would within reason be able to give you the answers to the questions you just asked. Loads and CGs vary from flight to flight. There's no way to predict it because there are so many variables.

The airlines have always used wags ... and these procedures are FAA approved. Until they pull out the scales and weigh every passenger and bag, this will continue to be the norm.

I believe the wags that are used are good enough to keep aircraft within tolerances. Yes, the FAA increased weight standards after B1900 crash. However, had the maintenance errors not occurred, it would not have crashed.