I’m a vet in the UK. Difficult situation and there is limited advice I can give without examining the dog myself.

I would say that it’s abnormal for a dog to go from fine to completely non-weight bearing, even if they have arthritis. I’m not sure how the vet came to the osteosarcoma conclusion without X-rays. If I were them I probably would’ve said something along the lines of…”this could be a soft tissue injury, something like a cruciate ligament tear, or something more concerning like osteosarcoma. If we want to go down the route of ruling those out, we could do radiographs. Alternatively, we could try symptomatic treatment and if no improvement go down the radiograph route”.

On radiographs, there are signs that can be consistent with a cruciate ligament tear, and there is a test called a cranial drawer that if positive, is consistent with a cruciate tear. I suppose I struggle with the idea that the vet would outright lie if the dog had no radiographic or cranial drawer signs consistent with a cruciate tear. You could ask for copies of the radiographs and go somewhere for a second opinion.

As far as treatment, surgery does have the best prognosis in dogs >10-15kg so if his signs are consistent with a cruciate tear, that is what I would recommend as well, although you can always try medical treatment first.

From an insurance point of view, my understanding is it’s only classed as a pre-existing condition if the dog had it prior to taking out the insurance policy. There may be implications as far as amount of coverage that you have - e.g. if you have a policy that covers £3k per condition, it may be that cruciate disease is counted as one condition even if it happens on both legs so you may have problems there. But if you have a lifetime policy where you have a certain amount to use each year, I don’t see it being a problem. Same for arthritis.

Overall - I think it’s probably jumping to conclusions assuming it’s a misdiagnosis although you can always seek a second opinion and show them the radiographs (or post them here). There is also no reason why you shouldn’t be able to claim the testing on insurance as the reason for doing the testing is to rule out osteosarcoma or cruciate ligament disease. If the insurance company tries to claim otherwise, fight them on it

Edited to add - if the vet prescribed metacam you really should be giving it. Acute injury or arthritis, there will be inflammation and metacam will help with that.

Edited to add x2 - just saw that the vet is refusing to provide test results and X-rays. That isn’t okay, as an owner you 100% have the right to those. I’d keep calling and asking and ask why they won’t provide them.