how does the following (taken from the point-by-point summary) answer the question of the differences between midoraisa and midarabannan on bedikos and chakiros:
1. Bedikos are also mid'Oraisa. Why is their law different?
(g) Answer (Rav Kahana and Rav Safra): If a witness cannot answer a Chakirah, he cannot be Huzam (since he did not specify when or where he saw the testimony), therefore his testimony is disqualified;
1. Even if he cannot answer Bedikos, he can be Huzam.
mechael benton, golusville- usa
If I understand your question correctly, the answer is based on the principle that "any testimony which is not subject to Hazamah is not valid." Accordingly, witnesses who cannot answer the Chakiros cannot be rendered Edim Zomemim, since Hazamah depends on the exact time and location at which the event took place, according to the witnesses' testimony. In contrast, if the witnesses are unable to answer the Bedikos, they still can rendered Edim Zomemim, and thus their testimony is valid.
B'Virchas Kol Tuv,
Eliezer Chrysler