Hello,
These are all questions to ask your vet. Without knowing specifically what the diagnosis is (or specifically what type of cancer) and what kind of fluid this is it is hard to predict outcome abd treatment options. Also the steroid can influence patient response. Cancer is defined as aberrant cell growth so it is by definition abnormal activity and asking to know and understand anything acting abnormally is also hard to predict. You have decided to keep her comfortable while declining the work up so we have a hard time answering specific questions about what to expect and do bed sue we do t have enough information to be able to answer questions like these.
Pain is impossibly to answer without seeing your pet in person and doing an exam to look for the signs that might indicate pain. Also we don’t have great long term options for treating pain in cats. Pred has some properties to help.
It sounds like you are doing the best you can with limited ability to diagnose which is the huge majority of cases I see. Keep doing what you can and it’s ok to seek multiple opinions.
Good luck.
Is it possible a mobile vet could come to you for this? Might be easier than trying to get them in carriers.