This species of test would also need a multi-day turnaround period on each image. And/or a video stream of the work being drawn.

"Changes" are an interesting one, honestly as a professional artist who has to pay her rent, there is certain complexity of change beyond which I am likely to say "look, we're going to need to renegotiate the budget on this if you want this much of a change from the sketch you already approved", or even "no".