Wow, first I just want to say how amazing you are for taking care of this sweet girl.
Regarding the diarrhea, I wonder if she perhaps has some sort of food intolerance or inflammatory bowel disease, and that's why you see improvement while on the metronidazole. Unfortunately it sounds like it's only suppressing the symptoms and not actually resolving the source of the problem. As well, long term use of metronidazole has its own set of side effects, some of them neurological. I have no idea if that would result in the random acts of peeing that you're starting to see, or if it's a completely unrelated issue.
Some questions: what does your girl eat, and how long has she been eating it? Has an "elimination diet" ever been tried? (that's where you exclusively feed a single novel protein source like venison or rabbit for a period of time, usually a few months, and eliminate all hyper-allergenic grains like corn, wheat, soy etc.). If not, that might be something to consider.
Other more holistic things to think about are slippery elm bark powder to help firm up her stool, and also the use of a high potency, multi-strain probiotic (something from a human health food store, not the vet).
Speaking of holistic, it would certainly be ideal if you could take her to a holistic vet for a consult. I realize that cost is an issue, and holistic vets are often initially more expensive than conventional vets, but over the long term a good one can actually be quite a bit cheaper.
Either way, I think a urinalysis would be a good starting point and it shouldn't be terribly expensive. A senior blood panel could also provide some clues.
I really hope you can figure out what's going on with your doggie. In the meantime, would putting puppy pads down in various places around the house help?