Awase by Lewis Bernaldo de Quiros

Yes I agree that 'kata' training the way we practice is about developing non-resistant relationship both as nage (not resisting but blending with the attack) and as uke (not resisting but blending with the technique) and in that sense the training is an 'agreement' that allows us to deepen and develop in to the principles.
But if  nage is able top absorb the  attack but uke becomes stiff and defensive in response to the technique, the technique will still 'work' but uke will be unable tp absorb and take the ukemi (ie receive) and will be injured though his own resistance (should nage choose to follow through with connecting to uke's center, which in practice he should not!).

So the training format requires understanding. When a technique 'doesn't work' is it because?

1. It is simply being executed poorly
2. Uke in 'knowing what the agreed technique' is has blocked it in some way (consciously or unconsciously)
3. Both of the above

In practice in the format that we use, the more common problem is that the techniques are being execute poorly in terms of appropriate connection and angles, yet uke falls anyway, even into the dan ranks!
What I teach my seniors (dans only) is that they should take the ukemi from a technique if it is minimally effective only, (but they should never block a technique!). With beginners (for a long time) the techniques do not work and seniors must 'guide' the junior nage into the correct technical relationship though his ukemi. This is why traditionally in many martial arts schools in Japan the uke is always the senior. The more senior the practitioners are, the less they should 'forgive' each other - without going into competing testing or blocking in any way. In this way we can actually get real feedback about what is relationally appropriate and what not. As Peter Ralston put it - our practice needs to be based on BRUTAL HONESTY.

This is why the martial aspect (Budo) needs to remain at the center of the practice: what functionally works and what does not. If we lose sight of this, Aikido becomes something else. Valid in another way perhaps but as far as developing REAL  'grace under pressure' or an authentic 'non-resistant relationship' in the face of conflict and determined opposition, of little use in my opinion.

If spiritual inquiry is about THE TRUTH and Aikido is a practice in line with spiritual inquiry then the practice should give me real feedback and reflection as to what is actually the case and not lead me into fantasy realms where my ability and understanding are vaguely related to what is actually relationally the case.  

For me, cooperation in practice means that uke is committed and real in both his attack and response. And for this he needs to be supple and responsive so in this I am in full agreement with you. 
But his responsiveness needs to be real, or at least have an orientation towards being real - and this is the real hard part (or rather, challenging part) to develop in Aikido: the embodiment of appropriate non resistant responsiveness (ukemi).

Lewis Bernaldo de Quiros