I am sorry to hear about your cat. I think it is a hard decision, adding to your already stressed and busy life. It is a pity we do not have gardens here, then the kitty would be OKish.
One more thing, are you sure she is OK, that the poop is not related to seizures or anything? When my Toto has accidents in the house, it is mostly because he had a seizure and I did not notice, rather than the fact that he is 17 years old.
Is there some way to make the litterbox more accesible? Cats are really clean, she can't be happy spilling her stuff on the floor, so you got her on your side for that.
I use vinegar for urine spills on clothing/linens -I soak the offended cloth in water and white vinegar for a while, voila- and I buy an organic cleaner at CitySuper/my local pet store that is absolutely fantastic with pet smells/stains/spills on the floor. One spritz and it is gone. And I have this impossible vinyll flooring which absorbs dirt and soaks up urine. I am also quite concerned about dissenfecting, if you'd like to try my steam cleaner you're more than welcome. Just remember not to use Clorox or bleach. I use baking soda for hairball removal.
here this one: http://www.simplegreen.com/products_pet_cat.php
On a personal note, it is difficult to tell when enough is enough. I wish my Mom would have put my cat down at 22 years old, not because she was old, but because she had a brain tumor. Muy Mom did not have the guts and had the cat ended up dragging itself around and agonizing for 3 days. Not what she deserved after being such a faithful companion but my Mom is not someone you could argue with.
Another friend had a cat also with a brain tumor nad had to make that choice. he was weak and miserable, even though he was younger, his quality of life was compromised. Even afterwards, she second guessed herself, as another vet told her there could be still more treatments to be tried. But those were a long shot. Kitty deserved to be gone with dignity and in peace.