I find it difficult to believe that a spring loaded throttle cannot be
modified with another spring! (This may be a liability issue when
modifying the original vehicle design.) Have them show you the throttle
spring so YOU can have it changed by a sympathetic mechanic.

The official fix may be to apply a lever to the stiff throttle so a
longer travel is substituted for the overly stiff spring.

I am assuming that you are using hand control for accelerator. Since it is a
mechanical device based on leverage, perhap they can modify the hand level
or custom make it so you have more leverage power. Call up the manufacturer
and discuss this with them.

My diesel pedal was very hard to depress when I purchased my car
recently. Very hard. I purchased a can of silicone spray lubricant and
applied some to the back of the accelerator pedal and it feels more
like a gas engine pedal.
On my car, the problem is not a spring on the pedal, but the fact that
there is very little mechanical advantage from the pedal to the
injection system. Although, I am willing to bet there is a way to get
the cruise control to control the accelerator manually like the foot
pedal would. However, that may be more difficult to figure out than
just adding a better mechanical advantage to the hand controls. I
looked at the site and it seems that the mechanical advantage could be
adjusted on those controls. If not, then look at another manufacturer.