Bladder disease in older cats is quite a different issue than what we usually see in cats under 10. In these little old kitties, the problem is often really kidney stones; sometimes little pieces flake off and cause a lot of pain and inflammation on their way out. Just ask anyone who's ever passed a kidney stone!
True bacterial infections are also a lot more common in older cats, so it's not surprising that she had to have antibiotics. Many antibiotics also have anti-inflammatory properties and will reduce pain--which is why they "seem" to work even when there isn't an infection.
It's possible to have a stone of one type and crystals of a different type at the same time. So, I'm a little concerned about the s/d, because kidney stones are universally calcium oxalate, and s/d can increase the formation of oxalate stones. It should never be used long term; but it sounds like it was only temporary for your cat.
The best preventative for any kind of stones or crystals is an all-wet, non-fish diet. These cats should not have any dry food at all. High protein is itself a diuretic and will keep the bladder flushed out so stone-forming elements don't have a chance to bind together. I recently had a chat with the top internal medicine specialist at Colorado State's vet school; he strongly recommends a high protein, high moisture diet (i.e., wet food) for older cats (as long as their kidneys are okay; if they're not, that's a whole 'nother story!).
Some supplements that are sometimes helpful including cranberry extract (250 mg per day), MSM (200 mg per day), glucosamine sulfate (250 mg per day), slippery elm (if you get capsules, give one capsule twice a day; open it up and mix it with a little cool water, and mix with wet food), and Renafood by Standard Process (sells to health professionals only; call 1-800-558-8740 to find a distributor in your area).
I'd also be thinking flower essences for stress, since that's a big component of most bladder issues.
Hope that helps!