I'm so very sorry to hear about Rosie, what a shock for you all. I don't have any experience of chemo I'm afraid so don't know what to advise, I know lots of people on here have had to make similar decisians about what is best for their cocker and when is the right time to say goodbye
its heartbreaking but such a personal decisian.
I don't think you would be selfish in the slightest for wanting to keep Rosie with you for as long as possible, any one of us would want the same. I suppose the question is what would her quality of life be for those 6 months or so? My personal veiw is I wouldn't want one of my dogs to undergo anything that made them feel any more unwell, as a human we understand that drugs are to prolong life so put up with side effects, obviously you can't explain that to a dog. However I am making that desician now when I am not in a situation like you, so I honestly can't say how I would react if I had to make the decisian, as I think emotions take over and all we want is to keep them with as as long as possible
I think whatever you and your family decide for Rosie will be the right thing to do - there really isn't a right or wrong in situations like this you have to go with your instincts and take it day to day and see how Rosie is.
Thinking of you and Rosie and sending much love