Hello again Geogia,
Regarding Bossy, I have worked out an answer for you below ...
1. Abdominal tumours are very common in older cats and sadly it is a common cause of death, surgery is rarely an option because removing the tumour itself may not achieve much as by the time you can palpate the lump in the abdomen the cancer itself is likely to be widespread in the cat's lymphatic system. In a cat of sixteen I would normally personally dissuade owners from any sort of surgery, but I would use exactly the same Vit B12 steroid cocktail your vet uses as this can perk these cats up and raise their quality of life for a short time, but usually then it will become ineffective and if at that point the cat is obviously suffering then that is the time to have your vet quietly and gently put Bossy to sleep. Sadly it would be rare for her to slip away on her own without going through a period of suffering. Sorry to so blunt ...
So on answer to your question all you can hope to do to help her is just to keep her comfortable and reassured for now and have your vet give the steroid B12 cocktail if she declines and keep your vet in the loop.
2. When to make the final decision? : Well over the years I have often been asked when is the right time to put a cat to sleep, there is no one easy answer for every case. But whatever condition your pet is suffering from at the end of the day you know Bossy best and you will know if she is suffering unduly. My experience is that if a client is asking this question then there are still good and bad points to weigh up, so all you can really do is to continue to weigh up these good and bad points, basically assess the quality of her life on a day by day basis, you know and obviously care for her so only you could make this decision.
Sooner or later a caring owner like yourself will just wake up one day, take a look at Bossy and say enough is enough. On that day follow your instincts, give your vet a call and make the arrangement to quietly and gently put her to sleep, almost certainly you will be doing the right thing ...
Also I should be available online for you the next four or five hours today so please do not hesitate to get straight back to me if her condition changes in any way or you would like to talk things over with me further.
Please be kind enough to rate my service to you after we have concluded our dialogue, such feedback helps me maintain the quality of my answers.