Pete, does your mom have a chronic lung problem such as COPD or emphysema? As far as the oxygen is concerned, don't be too worried when they change the methods of delivery. She should be on something that is monitoring the levels of oxygen in her body and will be adjusted accordingly. Something as simple as eating or more complicated such as suctioning can drop oxygen levels and may result her needing the mask. Of course it is best not to need these things but delivering higher levels of oxygen is a continually ongoing adjustment with people with respiratory problems.
The MRSA is a concern because of it is difficult to treat. Please be sure that everyone that comes into contact with your mother washes their hands. I can NOT emphasize this enough. MAKE EVERYONE WASH THEIR HANDS!!! And not just a quick rinse, they should be scrubbing to the entire first verse of Yankee Doodle Dandy.
The number of people who die from hospital acquired infections, almost always caused by poor hand washing techniques, is enormous. If someone can not be with her 24 hours a day - post a note over her bed. Make sure she is aware and makes everyone wash before they touch her. If they won't, send them away. Call the supervisor or the infection control nurse because this is vitally important and is a basic policy of any hospital.
But it seems as though your mother's spunk has pulled her through another difficult time. I am thinking about her and your family and hoping that she comes home as soon as possible.