You have picked an interesting topic to discuss on an open forum.
The short answer is....... call in sick when you are not fit for duty.
As far as using MUS, depending on how much sick bank you have, how often you are ill, and other factors; MUS could be a useful tool to re fill your bank with trips that are very attractive but you otherwise might not be able to pick with other M/U options.
I don't have an answer on how long your MUS bank lasts, my guess is a calendar year.