Ok, let's start bottom up -
a) Iran hit US with BMs, true. But point is, it was more for the public consumption than for the actual intent of hitting US. US already had Intel and they probably hit an airbase which was already vacated. Hence, no retaliation.
b) DF - 21 is deployed against carriers. If used by China on US (United States) during an active war, you think US will wait for the missiles to come down to judge if it was a nuke or not? I honestly don't think so. Having and using are two very different ball game.
c) As far as I know, and correct me if I am wrong, it is not possible to mount a nuke on a rocket artillery of range say 400 KMs. Also, the trajectory of the rockets will be a big give away as to its nature. For a SRBM, it will still follow a ballistic trajectory, which is not the case for rocket artillery. So, you mean, it is not possible to distinguish between a BM and rocket artillery using satellite and/or radar?
d) In a conflict, I don't think decisions are taken like - we hit a BM with conventional w/h, it's upto the adversary to decide if they want to go nuclear or not. I don't think any responsible country would like to create an ambiguity that can be fatal for both and that too unnecessarily. The MAD scenario is bad for both, and that's not all, it's bad for generations to come.
Lastly, nirbhay can hit the targets easily. Also, yes, it is true that subsonic missile can be intercepted, it is also true that it can go below radar in a terrain hugging mode. Tomahawk has ample such instances. And no, not comparing Nirbhay with Tomahawk directly.
