Power wheelchairs
look much easier to drive than they are. An
ECV is quite easy and intuitive - there is one throttle for going forwards and a separate throttle in a different place for going backwards. To steer, you turn the handlebar/tiller in one direction or the other; it is very much like steering a bike (and not that more difficult than steering a car, for someone who hasn't ridden a bike in years).
A power wheelchair uses a joystick that controls speed, direction forward and backward and turning -
ALL AT THE SAME TIME WITH THE SAME JOYSTICK. Because of that, it takes quite a bit longer to get the knack of driving one. My DD drove into lots of things before she got good with driving her power wheelchair. And actually, when we saw a new orthopedist, he asked how many times she had broken her toes, since most of his patients have broken toes driving into things with their power wheelchairs (for the record, my DD has not broken any toes).
So, most of the companies will only rent power wheelchairs to people who already have considerable experience using them (people like my DD, who have one at home, but don't want to travel with it).
Also, the power wheelchairs that companies do rent out don't have elevating footrests, so you would be in the same boat as far as not being able to put her foot up.