Andy, I know you said it twice already but I'll say it again for you. He has not yet run the engine on this plane.
I think you answered your own question by stating that the servo arm bends. It should not bend even with a standard servo arm. I think you have a bind somewhere. I would disconnect at the servo end and operate by hand and see how it moves. It should move easily and not require much torque. I use old S148s on throttles.
I don't know how stiff the return spring is on your engine. You can disconnect it, but not remove it or stretch it out some so it is not as tight.
You don't need much servo travel to operate a throttle on gas engines. Mine are almost always on the first or second inside hole.
There is not much rpm change anyway from half butterfly to full open on gas engines anyway so use care when setting your end points on throttle as you really don't need to "slam" it full open.