Your description of Lia's limp and overall behavior sounds a lot like Sweep. I will spare you all the details of our long saga, but it started with a limp upon rising in late summer 2015, shortly before she turned 6. Over the course of a year or so, we did several rounds of x-rays, acupuncture, chiropractic, Chinese herbs, myriad joint supplements, months of leash walking, consults with orthopedic specialists, laser therapy, massage, and probably other stuff I'm forgetting. Nothing has ever showed on an x-ray, but eventually our second orthopedic specialist was able to isolate the pain response to a weight-bearing toe on her left front foot. The best guess is that it's an old racing injury that has developed arthritis over time.
The hardest part for us was determining where it hurt specifically. I'd definitely consider a consult with an orthopedic specialist if there's one near you, and another set of x-rays if it's been a while since the first set. If you can isolate the issue, you can try more targeted therapies like cold laser, acupuncture, chiropractic, and therapeutic massage. That said...
Every dog is different and people have justifiably strong opinions about NSAIDs, but for us Previcox has been the most effective treatment. Sweep has been off and on it for almost two years. Sometimes she only needs it every other day or so, sometimes daily, sometimes not at all. (I haven't figured out the pattern yet, but I suspect humidity and activity level affect it most.) And her bloodwork is still perfect (knock on wood). So I would try the Rimadyl if you haven't done so. If it works, your girl gets some relief and you can go from there. If it doesn't, that's diagnostically useful too.
The second most effective thing has been massaging the toe; the physical therapist taught us a couple of specific techniques, and Sweep obliges by roaching and sticking her feet in our faces. Your vet might have a recommendation for a PT in your area.
I know it's a worrisome and frustrating process, and wish you the best of luck figuring this out.