You seem to have two questions: can we manually change an orders status, and can we change an order item's status, separate from it's parent order.
For your first question:
Yes, you can manually change order status; I believe this can be done when viewing the order.
For your second question:
Unfortunately, no. Magento does not natively possess the capabilities to specify order item state/status; rather, the system implements state/status processing on an order-by-order level.
You probably noticed that I used the term state/status rather than just status. You may already know this, but Magento differentiates between the two: order state is used by the system internally to track the processing flow of an order, while order status is used by merchants to track the processing of an order.
While Magento does not natively possess the capabilities to specify order item state/status, you could have a custom extension developed that does this; however, the cost is likely to be very high to integrate it fully into the system, so I'd encourage you to ensure that you really need the feature.