Apologies, I have been calling poor Trio a boy! In this scenario, it is actually better she is female as females are less prone to blockages, so that's a plus.
It's worth keeping in mind that a cat who has previously formed stones is more likely to form them again so it would certainly be worth ensuring this is not the case by performing some basic imaging (x-ray and/or ultrasound).
It's great she is on a urinary diet as this can help keep the urinary pH at the correct level and prevent crystal and stone formation.
Certainly cats are Very sensitive little souls and the changes that you have mentioned could be enough to cause a bout of cystitis.
Frequent urination and over-grooming tells us that her bladder is likely inflamed. This may be due to crystals, stones, infection or something less common (e.g. a bladder tumour, kidney disease).
Ideally, she would be examined by a vet as they can give her an all over check and assess her bladder. They would also be able to perform a simple urine test to check for many of the conditions mentioned above. If you bring a urine sample with you, this would speed the process up.
Given her age, a basic blood test could be a good idea as it can check her kidney function and also give us a baseline if we start her on any meds.
For most, a short course of anti-inflammatories should settle things down and stop the over-grooming. If we find an infection is present, antibiotics will be needed for a few weeks.
I would be consistent with the urinary diet and try to increase water intake as much as possible- add water to the food, offer rain water, use a water fountain etc.
Also make sure the tray is always clean and accessible, with no-one around to disturb her. If the new pet is a cat or if there are other cats, there should be 1.5 litter trays per cat (so 2 for one cat and 3 for 2 cats).
This website has some really good advice when it comes to managing urinary issues:https://icatcare.org/advice/feline-lower-urinary-tract-disease-flutd/