Forgive me for being dumb, but why all this fuss/paperwork for a throttle on a legal ebike?
I had a legal e-bike in 2011ish and it had a throttle. I never used it. I always had e-assist on Max setting. If it's legal (250W and limited to 25km/h) then the presence of a throttle does not make any difference. Even on the flat, you can go faster pedalling than you can with WOT. Downhill, irrelevant, you'd cruise at over 15mph so e-assist deactivated. Uphill, combination of e-assist and pedalling so no need for throttle. And it wouldn't be comfy standing up and trying to hold a twist type throttle anyway. Throttle only up a hill = lucky to go 4mph if it'll even get up.
Either my £1500 Wisper was especially weak, or these newer legal bikes put out a lot more torque for their 250W power