Dough safety and viability after overproofing
It is safe to eat?  Almost certainly, especially if you bake it.  Your dough doesn't contain anything that will "go bad" in 15 hours at room temperature.  Many bread dough recipes containing only flour, water, salt, and yeast are left to proof at room temperature for 12-24 hours, though they generally start with a much smaller amount of yeast.

Could it "go bad"?  That would only be possible if one of your ingredients was already contaminated with significant amounts of bacteria, mold, etc.  If you saw weird things growing on the dough or it had a foul odor, I suppose that could be an indication that something was "bad," but none of that should be possible in 15 hours for normal bread dough, assuming your ingredients weren't contaminated to begin with.  (Under normal conditions, it will probably take a few days or more at room temperature for bread dough to start actually "going bad" and growing mold, etc.)

However, the bread dough has probably "gone bad" in the sense that the bread produced will likely be inferior in flavor and texture.  If you bake the dough "as is," it will likely collapse significantly in the oven and be rather dense.  Chances are the dough will taste a bit odd after baking -- overly "yeasty" or "beer-like," with some "off" flavors.  It won't be completely inedible, but it probably won't taste great.  Personally, I wouldn't waste my time doing that.



The above answer I believe covers the specific question, but what can one do in this circumstance to "save" the dough?

It is possible to try re-kneading it for a few minutes and see if it will rise some more (re-kneading will redistribute the yeast and allow them to perhaps find more food), but that seems unlikely after such a long proof with so much yeast initially.  

At this point, the likely only way to save it as bread would be to use it as a "pre-ferment" for another batch, that is to cut up the dough into pieces and mix into another batch of dough (perhaps tripling the overall batch size, while using no yeast or perhaps only a small amount).  Then let proof, divide, and bake.  But I personally wouldn't do this unless I were sure the dough didn't taste bad, because in your situation the dough might have acquired some less desirable flavors, and you'd be wasting more ingredients to produce bread that tastes a little "off."

If you were desperate to use the dough for something and didn't want to risk an even bigger batch of inferior bread, I'd knead the dough a bit, divide it up, and use some sort of fast cooking method, probably with some other food or flavorings where a significant rise isn't needed (e.g., pizza, flatbread, fried dough, etc.).