Brake testing

In your first question I'd agree. When I used to do those types of vehicles many years ago, VOSA used to advise me to get a weigh bridge ticket, which was easy for me because they were five minutes down the road. Once I'd got the weight I used to add between 140 - 180 kg to it as per the then testing standards. That was then used as my brake testing weight. 

In your second question I'd say it means...

When the park brake efficiency is not met, i.e. 16%, then any percentage less than 8% would suggest that the park brake efficiency is now less than the 50% of the required value and is considered dangerous. 

The other way to consider it is the way you have already interpreted it, however, the problem with that method is that you can't apply it to all vehicles, hence some stations can't test 4x4 in the rollers, and if the park brake is known to have an efficiency problem on one wheel, then a road test to check whether the vehicle pulls to one side while driven is carried out, but the weight of the said vehicle at about 20 MPH will prevent you from proving that, hence a pass and advise.