I've found at least some answers to the witness questions here. Relevant points:
Whether or not a witness has been interviewed or called as a witness by the other side, the prosecution and defence:
may interview each other's witnesses or prospective witnesses;
may take statements from those witnesses; and
must ensure that no attempt is made to persuade the witness to change his/her story.
Where it is intended to interview a witness for the other side, who has given evidence or who it is known will be giving evidence, it is wise to give notice to the other side stating:
that the interview is required;
the reasons for it; and
that a representative from the other side should be present.
A witness does not have to be interviewed; it is a matter for the witness, who can refuse. A witness may agree to be interviewed only at a police station, in which case this facility should be made available.
So Helen's legal representatives could interview Henry. I wonder whether Knob would try to insist on being the "responsible adult"?