Is there an alternative therapy for osteosarcoma?

No. Osteosarcoma is a form of cancer and convnentional oncological therapy is the best option. There are however centers that focus on an integrative approach. Good luck to you.
Not without chemo. The standard treatment for osteosarcoma is chemotherapy and surgical removal of the tumor. Before chemotherapy was used, the surgery was amputation and chances of survival were about 10%. With modern chemotherapy, amputation is very rare and chances of survival are about 70%. Any alternative that doesn't involve chemotherapy significantly lowers the chances of cure.
No really. No alternative, holistic or naturopathic treatments have been shown to be as effective in treating osteosarcoma as surgery coupled with radiation or chemo. While no one can predict what an indivdual result will be, applied to a population as a whole, surgery and apropriate treatment for residual tumor is still the gold standard.
No. This used to be almost invariably fatal; a majority are now cured. If you do not follow the best evidence-based treatment, which will include surgery, radiation, and/or chemotherapy, you will pay for your mistake with your life. If you are approached by a "natural healer" or "conspiracy talker", be aware that when their remedies fail, they will blame you, ("You ate GMO food.") Don't be a sucker.