“Zaydun ḥāḍirun” asserts Zayd's presence without explicit emphasis, suitable for a listener with no prior position. “Inna Zaydan ḥāḍirun” adds emphasis for hesitation. “Wa-llāhi inna Zaydan la-ḥāḍirun” combines an oath, inna, and the emphatic lām for strong denial.
The distinction depends on the listener's state, not merely the number of words. A firmly denying listener may call for stronger confirmation; an informed listener may find repeated oaths unnecessary. Appropriateness is contextual.