It's not feasible for them to have killed Shirley off I don't think.
First they would have had to go to the trouble of keeping it all an absolute secret, and then they'd have to deal with the aftermath. Carl murdering someone puts a shelf life on his character straight away in normal circumstances. He's messing with Phil though, and Phil certainly wouldn't rest until Carl was dead.... Carl has so far done some pretty dodgy things but I reckon he has a lot of mileage yet.
Added to that, if they had wanted to kill Shirley off then the perfect opportunity to do that would have been in the car crash.
I really hope she's not dead, after a year of being a tiresome bore she has come alive again these past few weeks!