If you do copy them, you probably will have to use double polarization (lights and camera all polarized), and that may not remove all the glare from the tarnish. **warning: digital statements** I've scanned tarnished images using only the red channel to eliminate the bluish glare as much as possible, fixed them further in Photoshop, and then colorized them to a nice antique-y brown color. It works really well.
Yes, it is a "terminal condition" and you should make copies. The rate of deterioration varies widely depending on original materials and quality of original processing and subsequent display and storage. Black-and-white copy negatives are a very good, stable storage medium. Copy prints are good for display. Store the originals away in acid-free paper and archive-quality boxes in a cool, dry, dark place.