Progress. Some errors, and when deleting the last subform element, the delete doesn't take because it's empty.
Ah, so close to there, created a hidden FormSet element with formset name to pass to post, since 'FieldSet' names don't get submitted in PHP. Finally worked, but something about deleting a subelement when there are several causes a DB exception
Control/Element value being set twice, sometimes same, sometimes different (esp. after Post submission). About to refactor.