That's great news about your son. I hope everything works out with that.

As for everything else, I think I would find it difficult if my (theoretical) partner was open about their therapy if they had immense transference for their therapist. I would not make the demand that your husband did, as I wouldn't feel I had the right to make unilateral demands in such a way. However, if my partner made something "VERY symbolic of T and has the same look/feel as T’s office" for use as a "comfort item" between sessions, I would be extremely uncomfortable. I would prefer to be oblivious to this sort of thing, transitional objects, etc.

I don't really agree with your therapist's assessment that the best way of dealing with insecurity is with openness. I don't really think insecurity is something that should be managed by other people, but even if it were, I can't imagine how being open about this would diminish your husband's feelings of insecurity or jealousy. I say that because I think many people who weren't already insecure or jealous might become so with too many of this type of disclosure.