I don't think bitcoin's work function is based on graph isomorphism?

Yeah, I read more about it, only subgraph isomorphism is known to be NP-complete...of course the complexity of that will remain a mystery for a long time.

And finding a nonce that makes the block have a SHA256 with many leading zeroes is probably not an NP-hard problem.

It is, some people also tried SAT solvers for mining:

http://jheusser.github.io/2013/02/03/satcoin.html

Please look up the definition of NP-hardness again. If the problem

(#) := "finding a nonce that makes the block have a SHA256 with many leading zeroes"

was NP-hard, it would be an interesting idea to use a solver for (#) to solve e.g. SAT (not the other way round!).

TLDR: You talked about the wrong inclusion.

Not even related.