To be "aiki" one should at that point switch the technique to whatever the energetic situation calls for but in most dojos doing a technique which the teacher hadn't demonstrated isn't considered acceptable. You are supposed to do the technique demonstrated. This makes it all the more important that the partner deliver the type of attack for which the demonstrated technique is appropriate because to do anything other than that results in forcing the technique and complete lack of aiki.
There is the other side of over-investment of success into self-esteem. The above suggestion also has the unfortunate problem of people who have reached a certain level of proficiency but refuse to grow past it. I have seen good teachers demonstrate techniques in a way to challenge such surface-level understanding and rather than the "student" face the possibility that they don't everything already, they just switch techniques and blame the uke and the teacher - and basically anybody else as long as it's not them or their perfect technique.
My feeling is that training should be on the line between what you can do and what you can't. So at every level you should be successful only about 50% of the time.
As far as taking appropriate ukemi - well that's hard! I am just starting to really understand why Gleason sensei wanted me to move in some of the ways I move now. I used to think he wanted me to fall for him, but that wasn't right. Then I though he wanted me to resist him and test him in every way I could and that wasn't right either. The big open body - using your legs to keep your arms driving forward instead of contracting, keeping aware and totally responsive, looking for openings to enter and exploit, kind of attitude is difficult to describe. I'm told that some of the things I work on in ukemi are impossible - like keeping my energy consistent in all parts of my body while taking ukemi. I haven't stopped working on things like that yet. I guess I just need to arrive at that conclusion myself, and while I can't do it 100% I think I can get closer to 100% than I am now.
Anyway, I don't mean to steer the thread off topic. I think the main issue here is "trust building". Your relationship with your partner needs to be "trust building" and often it is "trust destroying" because you were inappropriately being "trust testing". Always be "trust building" between you and your partners, and then in turn help each other build trust in your technique and understanding of principles.