I understand how overwhelming this can be for you. To summarize, your Westie began having seizures 2 months ago. This is worrisome for brain tumor or, less commonly, adult onset epilepsy. Inappropriate urination and defecation as you're reported is commonly seen with cognitive dysfunction as might arise secondary to brain tumor as well as adult onset epilepsy particularly because she isn't completely controlled by her anticonvulsant medication. Which anticonvulsant(s) is she currently being administered?
Her urine is dilute...this can occur because she's drinking more which can be a side effect of phenobarbital administration, if her urinary tract infection involves her kidney(s), and many other metabolic disorders which cause polydipsia (increased thirst) and subsequent polyuria (increased volume of urine). Were there any other abnormal findings in her blood/urine tests which might indicate why she would be drinking excessively? Is she currently being treated with a systemic antibiotic? Please ask her vet if the beta hemolytic strep could represent a contaminant. There are more common bacteria found in the urinary tract of dogs. If the strep is considered important, she'll need 10-14 days of the appropriate antibiotic and then it's important to recheck a culture to make sure that her urinary tract has been sterilized. I understand that this will be expensive for you but unless infection is gone, we shouldn't consider "potty training" your dog as she was trained as a pup. Similarly, until her seizures are better controlled, we can't expect her to remember her potty training. Unfortunately, if a brain tumor is present, seizure control will be difficult, her mental faculties will be altered, and she's likely to continue inappropriately eliminating in your home.
Can you upload a copy of her test results to our conversation? I understand that you might not have a copy of the results at home but her vet should be able to give you one which you could scan into your computer and give me the link or you can photograph the page(s) and upload the images by using the paperclip icon in the toolbar above your message box (if you can see such an icon) or by using an external app such as imgur.com or dropbox.com.
Please respond with the additional information I need and further questions or concerns if you wish.