Lancslass.
Unfortunately I think you may be asking for too many 'must haves' and really are going to compromise on some features.
No legal crank drive or hub drive pedelec will go up a 12% hill without some rider input,you will need minimum 500 watt to achieve that, and you realise that is illegal as a bicycle.
The best legal hillclimbers are the BPM hub motored bikes such as the Woosh Big Bear,the Kudos Tornado,Sonata,Arriba and Rapide,the Oxygen bikes and KTM Panasonic hub bikes. I am told that the latest Kalkhoff crank drives are good up hills.
Throttles.....some of the Kudos bikes have take-off throttles ,working up to 4 mph,full speed throttles (up to 15 mph) will be illegal on bikes bought after 1 January 2016,unless type approved but as yet I cannot see how these e-bikes can be type approved. But do not be under the impression that with a legal e-bike you can twist and go up hills,without rider input,that is just not possible.
The Kudos Eiger is crank drive with a TCM motor,the motor is built at Tianjin university,I named it TCM whilst waiting for the ash cloud to go and sitting in Shanghai airport,this motor is not as powerful as the BPM hub,others use the same motor.
You started your post wanting Dutch style bikes but these tend to use the old SWX motor,Holland is flat so hill climbing is not a priority,these motors are not that powerful.
Hydraulic brakes require less effort to retard downhill.
OK,you need to look for 26" wheeled bikes,I assume step through,with a throttle,BPM motored,hydraulic brakes,probably a 46cm seat post....my Kudos Sonata would do most of your needs,but it has only a set off throttle,but you will still need some rider power input up steep hills....maybe others can recommend alternatives.
KudosDave