Two problems here. First, while it is true that the WC did not employ a full adversarial approach to truth-seeking, it is not the case that the WC placed no checks on its fact-finding process. For one thing, the WC engaged Walter E. Craig, then President of the American Bar Association, to advise the WC on whether its activities "conformed to the basic principles of American justice" (WCR xiv). Craig had the ability to cross-examine and recall witnesses, and he "participated fully and without limitation" in the process (xiv). His participation was agreed to by Marina Oswald.
Second, you beg the question whether a full adversarial approach would have yielded better, fuller, and fairer results. Had LHO lived, an adversarial trial would have enabled his counsel to keep lots of information away from the jury and therefore from the American public. Hearsay objections would have precluded many statements that were available to the WC. (Hearsay exclusions do not always assist the truth, which is one reason why there are so many exceptions to the hearsay rule.) LHO would also have invoked the spousal privilege to prevent testimony by Marina and to exclude many of her pretrial statements, as well as to exclude physical evidence that would have required her testimony for foundation and authentication. And so on. Would such evidentiary exclusions have resulted in a better understanding of the assassination by the American public, or would we have much less information than we have, whether or not LHO had been convicted?