Hi there, I'm Dr. Sara. Congratulations on the adoption of your new girl! What a blessing for her to be adopted even as a senior pet - thank you for giving her a great new home!
The estimated amount of water consumed in a day is about 60ml per kg of body weight, which for her is around 330ml. I actually suspect that the moist food and added water you are giving her is hitting that target. A dog that is otherwise healthy will not get dehydrated from neglecting to drink - they will self regulate and drink what they need, so I wouldn't worry about what she is or isn't drinking voluntarily.
You mentioned that she was on Royal Canin - is this the prescription urinary SO diet or a regular diet? Do you know if the stones were struvite, oxalate, or some other variety? All of them have the potential to recur, but each type has different prevention strategies. For instance, with struvite stones, prevention of bladder infections is the primary goal, and while diet can be helpful, it's not the most important factor.
If she is eating a prescription diet, keep in mind that the diet is specifically engineered to produce urine that falls within a narrow range of pH, concentration, and other attributes. Feeding her anything other than that specific diet, even treats or vegetables, can throw off all of those attributes and basically nullify all of the benefits of the diet.
Unfortunately, without knowing which type of stone she had, I can't make specific recommendations on some of your questions because the answers will differ based on the stone type. If you do know which type of stone, I'd be happy to offer more input!
Please let me know if you have any other specific questions :)