Is surgery ever appropriate for metastatic cancer?

Yes. Yes. In general surgery is not useful for metastatic (spread) cancer. However in oligo (few) metastatic cancer -- especially for breast, colon, melanoma cancers patients may be rendered stage 4 ned (no evidence of disease). This may improve local control and symptoms, survival, and in a few cases even lead to cure. Removing the primary tumor (kidney) in renal cell carcinoma is helpful in met rcc.
Yes. But it depends on the type of primary tumor and the extent of metastatic disease.