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.).
I have not found that there is a definitive answer, the most common I know is that it depends on what you like for a pizza dough. I know many people who use a common bread dough for pizza. I personally find that too "bready", but they like it that way. When I first learned, I was taught use a basic french bread recipe and add a couple Table Spoons of oil then allow to rise in cool environment rather than warm. Many use a higher gluten flour, but if you prefer a more cracker like thin crust then whole grain and a extra pinch a salt and immediately cook after rolling out with only one rise. If you like a thicker, chewy crust, then I suggest letting the dough do a second 10-15 minute rise after you have it ready for toppings but before you put them on. Most times what many of us consider a better pizza crust, regardless of if we like thin, thick, crispy, chewy, or some odd combo is one with smaller fermentation bubbles which calls for slower rising than bread. Higher gluten, a more elastic knead, a bit extra oil and a pinch of salt can all help get this. Starting with a regular break dough would restrict you down to only being able to go with a slower rise in my mind.
Best Answer
When I retard dough in the refrigerator and then plan to do a final proof in a proofing box, I tend to put it right in from the fridge. I don't know that there's any advantage to letting the dough come to room temperature first, other than making your final proof take longer (and there's no advantage to that, given that you've already developed flavor through retarding the dough).
In fact, for shaped rolls that have been retarded, I'd worry about taking too long for the final proof: I'd be afraid the rolls might deform with a longer proof and/or the outer skin of the dough would lose elasticity and cause the rolls to spread horizontally, rather than rise high during the bake.
So, for preshaped rolls, I'd put them in the proofing box right from the fridge.
If. on the other hand, you've retarded the dough in bulk or in rough rolls you plan to reshape before the final proof, then perhaps it would make sense to wait for the dough to come to room temperature -- then shape and do final proof. (Though in that case, you can also bring the dough to room temperature in a proofing box too if you like. I'd just wait to do final shaping when the dough is closer to room temp.)