I cant comprehend that there are Ts who don't already understand attachment theory, developmental trauma, and how this might manifest in therapy. It's like performing surgery without having fully studied anatomy & physiology.

Seems Ts also need basic training in neurobiology. If a client has regressed the way Seahorse is describing or is in the grip of overwhelming emotions, it would helpful if the T knew that trying to work through this rationally (boundaries, termination, intellectual insight) might be completely wrongheaded. Couple quotes on this (Bessel van der Kolk MD):

"When the alarm bell of the emotional brain keeps signaling that you are in danger, no amount of insight will silence it."

"For a hundred years or more, every textbook of psychology and psychotherapy has advised that some method of talking about distressing feelings can resolve them. However the experience of trauma itself gets in the way of being able to do that. No matter how much insight and understanding we develop, the rational brain is basically impotent to talk the emotional brain out of its reality."