It seems to me that it would be more meaningful to talk of spontaneity if we could come to a fundamental consensus. Is it:
1.) A muscle to be exercised? or a
2.) A switch to be flipped?
In nearly 30 years of both formal Zen and Aikido practice (not to mention 30 years as a professional skeptic, aka electrical engineer), it is absolutely clear to me that true spontaneity is not the former. That while jiyu waza may provide insight into spontaneity, it will never, on its own, actually develop spontaneity. Sorry.
Spontaneity is not the equivalent of a muscle. Jiyu waza can only ever develop familiarity, which, actually, is a 'muscle to be exercised'. And, unfortunately, many unsophisticated practitioners and observers will inevitably be seduced by advanced familiarity, which they will mistake for spontaneity.
Familiarity and advanced familiarity are perhaps difficult enough to train for some and a worthy endeavor. If a student learns to walk with better stability and balance, and more effective biomechanical structure, options open up for combat but also life in general. Reflexive movement that happens beneath conscious thought or planning can be trained, and it is not truly spontaneous. Ideas like spiraling to neutralize resistance while attacking the center are not truly spontaneous but can lead to two people moving together in a way that they never planned or intended to. The spontaneous creation of technique I think means the interplay of at least two people, but these people might be moving in explicitly trained ways.
For combat, moving from the center while attacking their center is not an unlimited set of variables; we would want to weed out spontaneous falling in the fetal position, or fainting, or stiffening up, or any number of other things.
If I understand you, Not breaking the flow might look spontaneous but is an understanding of the options available at a specific moment and feeling for uke's balance and movement and it is a quick decision. I still like it. I do not want a desire for spontaneous technique to mean training to be brainless.