Hi ElleTrixie, welcome to pets.ca!
Do you know if Dash's urine was sent out for a culture & sensitivity (usually takes a couple days to get the results back)? Can you get the results of the urinalysis?
See, the thing is, it's extremely rare for a cat under 10yrs of age to have a urinary tract
infection. This is more likely a case of urinary tract
inflammation, which means that the Clavamox is not only unnecessary, but potentially harmful. Vets have this rather annoying tendency to hand out antibiotics like candy, without doing the necessary lab work to confirm whether they're even needed.
In the meantime, it's definitely a good idea to counter the effects of the antibiotics since she's already on them, but yogurt probably isn't going to cut it (not enough probiotics in it). A better idea would be to get some probiotic capsules from a health food store - something with multiple, high-potency organisms, like
Natural Factors Ultimate Multi. You can mix just a pinch from the capsule in with some wet food a couple times a day.
Do you know what type of crystals Dash has? They're usually either struvite or calcium oxalate. Either way, the best food for both is a species appropriate
wet food diet (no kibble), and I'd suggest eliminating fish since that can aggravate bladder/urinary tract issues. Here is some good info for you on feline nutrition:
www.catinfo.org