I think the first thing to do is write to the PALS department of the hospital saying you have an official complaint and by all means include the chief exec in your letter but be aware that until it is classed as significan they may not be involved. It may also be a good idea to include the Medical Director (I don't know if all hospitals have these but we certainly do).
However... As a side note, 25 tumors in the brain is a sizeable number and I suspect your mother may have been suffering from symptons before the A&E visit no matter how minute or easily dismissable. She should certainly have had a scan (in my opinion) but I'm afraid the outcome would have been the same, that scale is only palliative treatable. Also, although you advised the doctor as to your mothers normal level of fitness, if there were no obvious symtons then it may be inexperience rather than a lapse in care that led to them sending your mother home.
Another thing to consider is, are you out to stop the same thing from happening to someone else or are you looking for someone to blame? I do not mean this in any kind of harsh way at all and I am not questioning your motives exactly. What I am questioning is that in your grief, are you looking for something else to focus on? By all means make your complaint, but make sure that you are allowing yourself to grieve too. You're very vulnerable when you're devastated and left footed in my experience.
I am truly sorry for your loss.
China