```python
def cheapest_pass_plan(trip_days: list, passes: list) -> dict:
    """
    Return the cheapest plan for covering all trip days with a combination of pass kinds.
    """
    if not trip_days or not passes:
        return {"total": 0, "purchases": []}

    # Validate inputs
    for day in trip_days:
        if not isinstance(day, int) or day <= 0:
            raise ValueError("Trip days must be positive integers.")
    
    for pass_ in passes:
        span = pass_.get('span', 0)
        cost = pass_.get('cost', 0)
        
        if not isinstance(span, int) or span <= 0:
            raise ValueError("Pass span must be a positive integer.")
        
        if not isinstance(cost, int) or cost < 0:
            raise ValueError("Pass cost must be a non-negative integer.")

    # Sort passes by cost and then span
    passes.sort(key=lambda p: (p['cost'], p['span']))

    total_cost = 0
    purchases = []
    trip_index = 0

    for pass_ in passes:
        start_day = max(trip_days[trip_index], next((day for day in range(trip_days[trip_index] + 1, len(trip_days)) if day < trip_days[trip_index] + pass['span']), default=trip_days[-1]))
        total_cost += pass['cost']
        purchases.append([start_day, pass['span']])
        trip_index = next((i for i in range(trip_index, len(trip_days)) if trip_days[i] >= start_day), len(trip_days))

    return {"total": total_cost, "purchases": purchases}
```