Will most IC's operate at 100C
To answer your question, what do I think? I would lay better than even odds that most samples of such a chip will be functional at 15°C over the guaranteed range, possibly not quite meeting specifications in one way or another. 

However, my guess and $2 will almost get you a coffee. If it's a student project or some hobby thing, and it doesn't really matter, YOLO. Be sure to use a battery that won't explode - many are rated for as low as 60°C. 

If you're going to exceed the recommendations in some way, try to stay conservative or near nominal in all other ways (keep the supply voltage regulated tightly near nominal, reduce clock speeds below the maximum if possible etc.) They don't really test the chips at high temperature, they infer operation at temperature extremes from the margins that they can measure an ambient. IOW, if a flip-flop will operate at 125MHz at 25°C, they know that they can guarantee a 100MHz spec (say) at 125°C (even at supply voltage extremes), so it passes. That's why "overclocking" sometimes works.