There is a qualitative difference between many PVS patients and the behavior exhibited in some of the videos. One could shoot days of footage of the other patients and not be able to get reactions as observed in these.
Having video to capture clearer bona fide efforts at communication would have been good, but Michael wouldn't allow such things. If he were acting in good faith, he would have sought to be (or at least act like he was) accommodating to the parents. Offer to let the parents shoot video if they like, but shoot his own copy with a wide-angle camera so as to be able to expose any trickery the parents might conceivably pull. Let Terri receive more testing. Let the parents try to give her food and water by mouth.
On the latter point, if giving food and water by mouth really were impossible, Michael would have benefitted from letting the parents try. Only if Terri was in fact able to accept food and water would such a concession be harmful to Michael. Given that it is unlawful to starve/dehydrate someone who is able to take food by mouth, I can see no legitimate basis for Michael's refusal. Illegitimate reasons are pretty easy to see, though.