is there a degree of relaxation one learns over time within the committed, intense attack, that permits the seamless transition to connected ukemi?
Relaxation may be the result of it, but I believe it starts in trust. You can't give a committed attack to someone without 1) the trust that they'll do their best to take care of you, and 2) the trust that you've developed sufficent ukemi skill to handle what comes out.
In the first case I don't mean they won't try to throw you hard, but more that they won't intentionally put some extra twist into things that makes uke's safety impossible - certain ways of doing some throws just can't be survived without injury.
Trusting yourself I think comes out of surviving situations that really pushed you. Always training at a comfortable level will never lead to that level of ukemi skill.
So, depending on what you do with your time it's possible that someone would never get there or get their quickly. Once you develop that trust though, your mind can clear of fear and your body can do what it's supposed to without interference.