Agree with C. B. Zeller on the plan document. Our provider also utilizes a box for whether a terminated participant is counted as employed for PS purposes if they terminated on the last day. I'd say, if there's nothing in your document on that, best to count as satisfying the last day condition since they technically were employed on the last day, even if only for a portion of that day.
Not so sure about the 5500, but if there's any doubt it's probably best to just count them as active.