The difference between "reflection" and "due consideration" is minimal, but present. To reflect means to think back on previous actions and analyze their effects. "To give something it's due..." means that something is owed, whether it's money, attention, or anything else that should be done to satisfy it. Hence, "due consideration" means that you have neglected to consider something, and you'll have the time to do so on your week off. "She decided on reflection" would mean that she considered his offer, and during this reflection she came to a decision. Again, "due consideration" means that she gave the offer the consideration that it required. The fifth sentence shares the same meaning as the third, but ends in "consideration," which changes the flow and rhyming scheme of the sentence to suit your needs.