First rule of Budo: protect yourself at all times. ALWAYS!
Aikido practise is inherently safe. When it is not, you are doing it wrong!
No one should make a point using pain/cause injury. Especially not the teacher. In case this happens do not train with that person, warn others about it or perhaps even leave!
In role of aite you allow tori to do the exercise, while protecting yourself at every step of the technique. It does not matter if tori is your teacher, an experienced student or novice. There is a risk that tori is too strong/too good for you to maintain control and you rely on your trust in tori. So know who you can trust, or do not train with them.
Accidents do happen, unfortunately, but they occur by chance not on purpose!
Again, I agree in the general case. In this specific case the shihan had seen the uke develop from gokyu up to sandan, and knew the limits. There was nothing fast.
If I'm uke, I essentially "let" the nage do the technique if he or she is doing it. I actually let them do it 'wrong' a few times while I'm figuring out if I'm able/qualified to help them. If I think I can help them, I do. If I don't think I know enough, I call the shihan or sensei over and ask why my partner is having so much trouble making me move. If it's a shihan doing the technique to me, I usually don't have much choice in the matter - OK, I don't have any choice in the matter. Hmm - THAT's what he's doing...
The only accidental ouch I've had when being thrown by a shihan, I was planted firmly on the ground with a kotegaeshi that had some pretty good air in it (flying lessons, Japanese style), which was just fine, but my lower leg, on the way to the mat, landed on someone's heel just as he was pushing off the ball of his foot. MAN that hurt.