Hi, Welcome to JustAnswer. I am a licensed veterinarian and I'm here to help you. Sorry to hear that Shelley is having this issue and causing you such a problem. All that you hve done so far is excellent, but I'd like to give you a few more tips to see if they do help. First of all you do need to thoroughly clean any areas where Shelley has urinated with an enzymatic cleanser like Nature's Recipe. This type of cleanser will not only remove the visible evidence, but will also remove the odor. I have found that you have to thoroughly soak any area, so don't be like me a try to save money by using less. . .it just doesn't work very well then!
Since you have 2 cats in your home, the recommendation is that you have 3 litter boxes in various quiet but easily accessible parts of your home. You're already keeping the litter meticulously clean so this is great, and you haven't changed the type of litter or the type of litter boxes, and this is also excellent. . .cats can be very picky.
To ease another part of your kitties' relationship, I'd feed them twice daily in separate rooms where they can't even see each other. Give them about 15 to 30 minutes to eat, then pick up the food. This way, they won't feel threatened over their food at all.
Having one or two kitty trees in your home would probably help. These are not only entertaining and enriching to their environment, they also offer high platforms on which the cats can lay. This is usually very calming to cats and may help Shelley's relationship with your other cat, as well as decreasing her inappropriate urination and defecation.
Finally, I know you said that your vet has checked Shelley's health, but has he done a blood profile and a urinalysis to check for the several common diseases that can affect our older cats? These diseases are hyperthyroidism, diabetes, kidney disease, lower urinary tract disease (affecting the bladder), and sometimes Cushing's disease. If Shelley has any of these, then her behavior won't be modifiable until she managed or cured. You also said that you have tried medications for her. Which ones have you tried, and have your tried at least several weeks of fluoxetine or Reconcile? I've experienced personally, and read research articles that this drug can be quite effective and safe.
If you have any more questions or concerns, please don't hesitate to reply.
I have used various enzymatic cleansers such as Nature's Miracle, etc, as well as just replacing soiled cushions (and furniture).
Shelley has been on fluoxetine yet without improvement. The vet kept her for two days to test blood & urinalysis, she said the issue was not physical yet only behavioural.
I have not tried feeding the cats in separate rooms...will do so starting today.