With billions in the bank, ISIS can buy many more that one nuke. If they buy more than one, then using more than one is as easy as using only one. Expect multiple sunrises ...
Nukes have specific signatures and the country of origin can be determined rather quickly.
Yes, I’m aware of that part, but how to prove it was ISIL and not the DPRK (who use Pakistani materials), Pakistan itself, Saudi Arabia or just a rogue group who set it off?